Dear @ctgraham and others!
Thank you @ctgraham for your help an patience!
I have solved this issue. I added mod_rewrite logging and got this error log when approaching the submissions page:
[Wed Feb 20 07:59:29.400006 2019] [rewrite:trace3] [pid 9527] mod_rewrite.c(470): [client x.x.x.x:3883] x.x.x.x - - [mydomain.se/sid#55cd7477b9b8][rid#55cd74992b80/initial] [perdir /var/www/html/] add path info postfix: /var/www/html/mydomain -> /var/www/html/mydomain/api/v1/_submissions, referer: https://mydomain.se/submissions
[Wed Feb 20 07:59:29.400075 2019] [rewrite:trace3] [pid 9527] mod_rewrite.c(470): [client x.x.x.x:3883] x.x.x.x - - [mydomain.se/sid#55cd7477b9b8][rid#55cd74992b80/initial] [perdir /var/www/html/] strip per-dir prefix: /var/www/html/mydomain/api/v1/_submissions -> mydomain/api/v1/_submissions, referer: https://mydomain.se/submissions
[Wed Feb 20 07:59:29.400083 2019] [rewrite:trace3] [pid 9527] mod_rewrite.c(470): [client x.x.x.x:3883] x.x.x.x - - [mydomain.se/sid#55cd7477b9b8][rid#55cd74992b80/initial] [perdir /var/www/html/] applying pattern '^(.*)$' to uri 'mydomain/api/v1/_submissions', referer: https://mydomain.se/submissions
[Wed Feb 20 07:59:29.400123 2019] [rewrite:trace4] [pid 9527] mod_rewrite.c(470): [client x.x.x.x:3883] x.x.x.x - - [mydomain.se/sid#55cd7477b9b8][rid#55cd74992b80/initial] [perdir /var/www/html/] RewriteCond: input='/mydomain/api/v1/_submissions' pattern='!/index.php/' => matched, referer: https://mydomain.se/submissions
[Wed Feb 20 07:59:29.400137 2019] [rewrite:trace4] [pid 9527] mod_rewrite.c(470): [client x.x.x.x:3883] x.x.x.x - - [mydomain.se/sid#55cd7477b9b8][rid#55cd74992b80/initial] [perdir /var/www/html/] RewriteCond: input='/var/www/html//mydomain/api/v1/_submissions' pattern='!-f' => matched, referer: https://mydomain.se/submissions
[Wed Feb 20 07:59:29.400146 2019] [rewrite:trace2] [pid 9527] mod_rewrite.c(470): [client x.x.x.x:3883] x.x.x.x - - [mydomain.se/sid#55cd7477b9b8][rid#55cd74992b80/initial] [perdir /var/www/html/] rewrite 'mydomain/api/v1/_submissions' -> 'index.php/mydomain/mydomain/api/v1/_submissions', referer: https://mydomain.se/submissions
...
It looks like the Rewrite-rules are adding “mydomain” to the URI multiple times.
I modified my apache conf-file by adding a RewriteCond
to look for “mydomain” in the address.
Now my .conf file looks like this:
<VirtualHost *:443>
ServerName mydomain.se
DocumentRoot "/var/www/html/"
<Directory /var/www/html>
AllowOverride None
Options Indexes FollowSymLinks MultiViews
Require all granted
DirectoryIndex index.php
RewriteEngine on
RewriteBase /
# If the URI already has index.php in it, don't change it
RewriteCond %{REQUEST_URI} !/index.php/
RewriteCond %{REQUEST_URI} !/mydomain/
# Skip existing directories if Apache 2.2 or later
#RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
# Skip existing files if Apache 2.2 or later
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
# Rewrite all other requests to a specific journal in OJS
RewriteRule ^(.*)$ index.php/mydomain/$1 [QSA,L]
# If the URI already has index.php in it, don't change it
RewriteCond %{REQUEST_URI} !/index.php/
RewriteCond %{REQUEST_URI} /mydomain/
# Skip existing directories if Apache 2.2 or later
#RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-d
# Skip existing files if Apache 2.2 or later
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
# Rewrite all other requests to a specific journal in OJS
RewriteRule ^mydomain(.*)$ index.php/mydomain/$1 [QSA,L]
</Directory>
## Logging
ErrorLog "/var/log/httpd/mydomain.se:443_error_ssl.log"
ServerSignature Off
CustomLog "/var/log/httpd/mydomain.se:443_access_ssl.log" combined
## Header rules
## as per http://httpd.apache.org/docs/2.2/mod/mod_headers.html#header
Header Set Strict-Transport-Security "max-age=31536000"
## SSL directives
...
</VirtualHost>