Univention UCS 4.4.7 - Collabora does not start in NC


being a ‘windows guy’ I clearly have simple troubles being now on a Linux machine. My setting:

  • UCS v4.4.7 installed (IP; internal name: ugolino.myfritz.net; external name: blahblahblah.myfritz.net

    • Ubuntu 20.04
    • NC 20.0.2
    • PHP 7.4.1
    • CODE 6.4.(?)
  • UCS runs on MS server 2019 in Hyper-V.

    • IIS is listening on port 80
    • IIS is configured as ReverseProxy to transfer all traffic assigned to blahblahblah.myfritz.net to the UCS machine.
  • Because I also need port 443 for SSH I have SSLH running on a different hyper-v machine.

    • SSLH is multiplexing https traffic to the UCS machine.

When I connect internally via “hps://" or externally via "hps://blahblahblah.myfritz.net/nextcloud/apps/dashboard/” I can access NC without problems and - for example - can also use the mail app.

Problem: However, when I click on a document and Collabora should start I receive the error message

Collabora Online could not be loaded - Please try again later

Using the administrator account and clicking on “Collabora online” I see that “use the built-in CODE - Collabora online developer edition” is selected and - with a red sign - I can read the error message “Could not connect to the Collabora Online server”.

In other words: CODE is installed, but I cannot access to it. I assume that this is due to my ‘strange’ (?) network configuration. But what do I have to do to use as well Collabora in my setting? I think that there are config files I have to modify - but which ones?