kolab(imap, chwala,seafile) and roundcube on different hosts

[Jochen Hein]

I have a kolab 3.4 server on Debian 8 with roundcube that is accessible
from the intranet. kolab_files is enabled and can access chwala/seafile
without problems.

I'd like to have the kolab server inaccessible from the
outside(internet), so I run a second roundcube instance on a different
VM. Authentication and reading/sending mails works fine, but using
kolab_files (pointing to seafile) gives mit an "An error occurred!
Connection Error (Failed to reach the server)". I've searched for hints
in the logs but was not successful.  Should that work?

/etc/roundcube/kolab_files.inc.php:
// URL of kolab-chwala installation
$config['kolab_files_url'] = 'https://kolab3.example.org/chwala/';

// List of files list columns. Available are: name, size, mtime, type
$config['kolab_files_list_cols'] = array('name', 'mtime', 'size');

// Name of the column to sort files list by
$config['kolab_files_sort_col'] = 'name';

// Order of the files list sort
$config['kolab_files_sort_order'] = 'asc';

// Number of concurent requests for searching and collections
listing. Default: 1
$config['kolab_files_search_threads'] = 1;

// seafile
$config['fileapi_backend'] = 'seafile';
//$config['fileapi_backend'] = 'kolab';
//$config['fileapi_drivers'] = array('seafile');
$config['fileapi_seafile_host'] = 'https://kolab3.example.org/seafile/';
$config['fileapi_seafile_ssl_verify_peer'] = false;
$config['fileapi_seafile_ssl_verify_host'] = false;
$config['fileapi_seafile_cache_ttl'] = '14d';
$config['fileapi_seafile_debug'] = true;


/etc/roundcube/config.inc.php:
$config['fileapi_backend'] = 'seafile';
//$config['fileapi_backend'] = 'kolab';
//$config['fileapi_drivers'] = array('seafile');
$config['fileapi_seafile_host'] = 'https://kolab3.jochen.org/seafile/';
$config['fileapi_seafile_ssl_verify_peer'] = false;
$config['fileapi_seafile_ssl_verify_host'] = false;
$config['fileapi_seafile_cache_ttl'] = '14d';
$config['fileapi_seafile_debug'] = true;

Any hints what I should try?

Jochen

-- 
The only problem with troubleshooting is that the trouble shoots back.