Hi Timo, 

I attach my Apache configuration. I'm using the web port (57772) of my IRIS instance, since I'm just testing the redirect on a development system.
Thank  you

# This is the main Apache HTTP server configuration file.  It contains the# configuration directives that give the server its instructions.# See <URL:http://httpd.apache.org/docs/2.4/> for detailed information.# In particular, see# <URL:http://httpd.apache.org/docs/2.4/mod/directives.html>#for a discussion of each configuration directive.## See the httpd.conf(5) man page for more information on this configuration,# and httpd.service(8) on using and configuring the httpd service.## Do NOT simply read the instructions in here without understanding# what they do.  They're here only as hints or reminders.  If you are unsure# consult the online docs. You have been warned.## Configuration and logfile names: If the filenames you specify for many# of the server's control files begin with "/" (or "drive:/"for Win32), the# server will use that explicit path.  If the filenames do *not* begin# with "/", the value of ServerRoot is prepended -- so 'log/access_log'# with ServerRoot set to '/www' will be interpreted by the# server as '/www/log/access_log', where as '/log/access_log' will be# interpreted as '/log/access_log'.
## ServerRoot: The top of the directory tree under which the server's# configuration, error, and log files are kept.## Do not add a slash at the end of the directory path.  If you point# ServerRoot at a non-local disk, be sure to specify a local disk on the# Mutex directive, if file-based mutexes are used.  If you wish to share the# same ServerRoot for multiple httpd daemons, you will need to change at# least PidFile.#
ServerRoot "/etc/httpd"

## Listen: Allows you to bind Apache to specific IP addresses and/or# ports, instead of the default. See also the <VirtualHost># directive.## Change this to Listen on specific IP addresses as shown below to# prevent Apache from glomming onto all bound IP addresses.##Listen 12.34.56.78:80
Listen 57772

## Dynamic Shared Object (DSO) Support## To be able to use the functionality of a module which was built as a DSO you# have to place corresponding `LoadModule' lines at this location so the# directives contained in it are actually available _before_ they are used.# Statically compiled modules (those listed by `httpd -l') do not need# to be loaded here.## Example:# LoadModule foo_module modules/mod_foo.so#
LoadModule dumpio_module modules/mod_dumpio.so
Include conf.modules.d/*.conf

## If you wish httpd to run as a different user or group, you must run# httpd as root initially and it will switch.## User/Group: The name (or #number) of the user/group to run httpd as.# It is usually good practice to create a dedicated user and group for# running httpd, as with most system services.#
User apache
Group apache

# 'Main' server configuration## The directives in this section set up the values used by the 'main'# server, which responds to any requests that aren't handled by a# <VirtualHost> definition.  These values also provide defaults for# any <VirtualHost> containers you may define later in the file.## All of these directives may appear inside <VirtualHost> containers,#inwhichcase these default settings will be overridden for the# virtual host being defined.#
## ServerAdmin: Your address, where problems with the server should be# e-mailed.  This address appears on some server-generated pages, such# as error documents.  e.g. admin@your-domain.com#
ServerAdmin root@localhost

## ServerName gives the name and port that the server uses to identify itself.# This can often be determined automatically, but we recommend you specify# it explicitly to prevent problems during startup.## If your host doesn't have a registered DNS name, enter its IP address here.##ServerName www.example.com:80
## Deny access to the entirety of your server's filesystem. You must# explicitly permit access to web content directories in other# <Directory> blocks below.#
<Directory />
    AllowOverride none
    Require all denied
</Directory>

## Note that from this point forward you must specifically allow# particular features to be enabled - so if something's not working as# you might expect, make sure that you have specifically enabled it# below.#
## DocumentRoot: The directory out of which you will serve your# documents. By default, all requests are taken from this directory, but# symbolic links and aliases may be used to point to other locations.#
DocumentRoot "/var/www/html"

## Relax access to content within /var/www.#
<Directory "/var/www">
    AllowOverride None
    # Allow open access:
    Require all granted
</Directory>

# Further relax access to the default document root:
<Directory "/var/www/html">
    #
    # Possible values for the Options directive are "None", "All",
    # or any combination of:
    #   Indexes Includes FollowSymLinks SymLinksifOwnerMatch ExecCGI MultiViews
    #
    # Note that "MultiViews" must be named *explicitly* --- "Options All"
    # doesn't give it to you.
    #
    # The Options directive is both complicated and important.  Please see
    # http://httpd.apache.org/docs/2.4/mod/core.html#options
    # for more information.
    #
    Options Indexes FollowSymLinks

    #
    # AllowOverride controls what directives may be placed in .htaccess files.
    # It can be "All", "None", or any combination of the keywords:
    #   Options FileInfo AuthConfig Limit
    #
    AllowOverride None

    #
    # Controls who can get stuff from this server.
    #
    Require all granted
</Directory>

## DirectoryIndex: sets the file that Apache will serve if a directory# is requested.#
<IfModule dir_module>
    DirectoryIndex index.html
</IfModule>

## The following lines prevent .htaccess and .htpasswd files from being# viewed by Web clients.#
<Files ".ht*">
    Require all denied
</Files>

## ErrorLog: The location of the error log file.# If you do not specify an ErrorLog directive within a <VirtualHost># container, error messages relating to that virtual host will be# logged here.  If you *do* define an error logfile for a <VirtualHost># container, that host's errors will be logged there and not here.#
ErrorLog "logs/error_log"

## LogLevel: Control the number of messages logged to the error_log.# Possible values include: debug, info, notice, warn, error, crit,# alert, emerg.#
LogLevel warn

<IfModule log_config_module>
    #
    # The following directives define some format nicknames for use with
    # a CustomLog directive (see below).
    #
    LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\"" combined
    LogFormat "%h %l %u %t \"%r\" %>s %b" common

    <IfModule logio_module>
       # You need to enable mod_logio.c to use %I and %O
       LogFormat "%h %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-Agent}i\" %I %O"
    </IfModule>

    # If you do not define any access logfiles within a <VirtualHost>
    # container, they will be logged here.  Contrariwise, if you *do*
    # define per-<VirtualHost> access logfiles, transactions will be
    # logged therein and *not* in this file.
    #
    #CustomLog "logs/access_log" common

    #
    # If you prefer a logfile with access, agent, and referer information
    # (Combined Logfile Format) you can use the following directive.
    #
    CustomLog "logs/access_log" combined
</IfModule>

<IfModule alias_module>
    #
    # Redirect: Allows you to tell clients about documents that used to
    # exist in your server's namespace, but do not anymore. The client
    # will make a new request for the document at its new location.
    # Example:
    # Redirect permanent /foo http://www.example.com/bar

    #
    # Alias: Maps web paths into filesystem paths and is used to
    # access content that does not live under the DocumentRoot.
    # Example:
    # Alias /webpath /full/filesystem/path
    #
    # If you include a trailing / on /webpath then the server will
    # require it to be present in the URL.  You will also likely
    # need to provide a <Directory> section to allow access to
    # the filesystem path.

    #
    # ScriptAlias: This controls which directories contain server scripts.
    # ScriptAliases are essentially the same as Aliases, except that
    # documents in the target directory are treated as applications and
    # run by the server when requested rather than as documents sent to the
    # client.  The same rules about trailing "/" apply to ScriptAlias
    # directives as to Alias.
    #
    ScriptAlias /cgi-bin/ "/var/www/cgi-bin/"

</IfModule>

##"/var/www/cgi-bin" should be changed to whatever your ScriptAliased# CGI directory exists, if you have that configured.#
<Directory "/var/www/cgi-bin">
    AllowOverride None
    Options None
    Require all granted
</Directory>

<IfModule mime_module>
    #
    # TypesConfig points to the file containing the list of mappings from
    # filename extension to MIME-type.
    #
    TypesConfig /etc/mime.types

    #
    # AddType allows you to add to or override the MIME configuration
    # file specified in TypesConfig for specific file types.
    #
    #AddType application/x-gzip .tgz
    #
    # AddEncoding allows you to have certain browsers uncompress
    # information on the fly. Note: Not all browsers support this.
    #
    #AddEncoding x-compress .Z
    #AddEncoding x-gzip .gz .tgz
    #
    # If the AddEncoding directives above are commented-out, then you
    # probably should define those extensions to indicate media types:
    #
    AddType application/x-compress .Z
    AddType application/x-gzip .gz .tgz

    #
    # AddHandler allows you to map certain file extensions to "handlers":
    # actions unrelated to filetype. These can be either built into the server
    # or added with the Action directive (see below)
    #
    # To use CGI scripts outside of ScriptAliased directories:
    # (You will also need to add "ExecCGI" to the "Options" directive.)
    #
    #AddHandler cgi-script .cgi

    # For type maps (negotiated resources):
    #AddHandler type-map var

    #
    # Filters allow you to process content before it is sent to the client.
    #
    # To parse .shtml files for server-side includes (SSI):
    # (You will also need to add "Includes" to the "Options" directive.)
    #
    AddType text/html .shtml
    AddOutputFilter INCLUDES .shtml
</IfModule>

## Specify a default charset for all content served; this enables# interpretation of all content as UTF-8 by default.  To use the# default browser choice (ISO-8859-1), or to allow the META tags#in HTML content to override this choice, comment out this# directive:#
AddDefaultCharset UTF-8

<IfModule mime_magic_module>
    #
    # The mod_mime_magic module allows the server to use various hints from the
    # contents of the file itself to determine its type.  The MIMEMagicFile
    # directive tells the module where the hint definitions are located.
    #
    MIMEMagicFile conf/magic
</IfModule>

## Customizable error responses come in three flavors:# 1) plain text 2) local redirects 3) external redirects## Some examples:#ErrorDocument 500 "The server made a boo boo."#ErrorDocument 404 /missing.html#ErrorDocument 404 "/cgi-bin/missing_handler.pl"#ErrorDocument 402 http://www.example.com/subscription_info.html#
## EnableMMAP and EnableSendfile: On systems that support it,# memory-mapping or the sendfile syscall may be used to deliver# files.  This usually improves server performance, but must# be turned off when serving from networked-mounted# filesystems or if support for these functions is otherwise# broken on your system.# Defaults if commented: EnableMMAP On, EnableSendfile Off##EnableMMAP off
EnableSendfile on

# Supplemental configuration## Load config files in the "/etc/httpd/conf.d" directory, if any.
IncludeOptional conf.d/*.conf
#### BEGIN-ApacheCSP-SECTION ####
LoadModule csp_module_sa "/opt/webgateway/bin/CSPa24.so"
CSPModulePath "/opt/webgateway/bin/"
CSPConfigPath "/opt/webgateway/bin/"

<Location "/csp/">
        CSP On
</Location>
<Location "/api/">
        CSP On
</Location>
<Location "/oauth2/">
        CSP On
</Location>
<Location "/isc/">
        CSP On
</Location>
<Location "/ui/">
        CSP On
</Location>

<Directory "/opt/webgateway/bin/">
        AllowOverride None
        Options None
        Require all granted
        <FilesMatch "\.(log|ini|pid|exe)$">
                Require all denied
        </FilesMatch>
</Directory>

Redirect /csp/docbook/ http://docs.intersystems.com/irislatest/csp/docbook/
#### END-ApacheCSP-SECTION ######## BEGIN-ApacheCSP-SECTION-IRISVECTDB ##### Note: IRISVECTDB reinstallation or upgrade may replace this section.
<Location /irisvectdb>
        CSP On
</Location>
Redirect /irisvectdb/csp/docbook/ http://docs.intersystems.com/irislatest/csp/docbook/
#### END-ApacheCSP-SECTION-IRISVECTDB ####

Thank you Enrico! The error arose from the permission of the web app. After changing it, it worked!