External Access to CHWALA fails

Hello,

we will access chwala from inside as well as from outside. From inside 
it works fine. From outside the access fails after the login screen. In 
our environment the DNS-name differs between internal and external 
access. For the outside access we have an other dns-name with a specific 
port (port forwording).
Access to the roundcube webmail works  fine from inside and outside, 
where the URL is the same, except chwala instead roundcubemail.

When I check to chwala-log-files, I see that the ssl-connection fails. 
The URL of the external address with the port is mentioned in the log 
file.

I tried to set the the paramter $config['kolab_files_server_url'] naming 
the internal DNS-name.

But this doesn't work.

Any idea?

Thank you

Roland