Another update.
I have found out that the upload of files within the page is hindered by this.
I have since changed the .htaccess to:
RewriteEngine On
RewriteBase /
# Bugfix for OJS 3.2.0-3: submissions API doesnt work with shortened URLs
RewriteRule ^api/v1(.*)$ index.php/journal/api/v1$1 [L,R=307]
# Rewrite URL to look cleaner
RewriteCond %{SERVER_NAME} ^www.server.com
RewriteCond %{REQUEST_URI} !/journal/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ index.php/journal/$1 [L]
which seems to fix the problem regarding POST data.