I've replaced the https_check() function with

As far as I can trace the calls back... our old friend api_url() is used to create the base URL and the scheme depends only on rcube_utils::https_check() and nothing else.
If the scheme "http" is used this can only mean, that rcube_utils::https_check() returned false - please correct me, if I'm wrong. Perhaps we should have a look at it next.

I've changed the file_api_url setting to some nonsense for testing purposes... Of course, it does not work, but the changed URL is passed along.

I've created a new text file and tried to open it. Same issue :-(

Is there anything I can do to help you to diagnose it?

I've set in kolab_files.inc.php

Yes, I've set use_https to true and applied the mentioned fixes. I'm sorry, but the issue persists :-/

Now the error is away. Thank you!

I've written out $_SERVER['SERVER_PORT'] ... it's port 80. I assume that rcube_utils::https_check() returns true, so $schema is "https", this leads to $port = 443 and to a true, when evaluating the new if-block. So the port is appended to the hostname.

I've done just copy & paste. The code of the resulting function is ...

Thank you for the fix, but it doesn't seem to work :-/

Setting use_https to true does has not the desired effect. The created URL now contains the scheme "https", but it still uses port 80. Other parts of the application now complain about errors like this one:

Oops, sorry...
Version: roundcubemail-plugins-kolab-3.3-1.10.el7.kolab_16.noarch