invalid base_url für m.homeserver

[nowatbuer]

I am having a problem with matrix when accessing matrix via firefox/nextcloud on debian. I only get the message:

Your element is configured incorrectly
Invalid base_url for m.homeserver

I can call matrix directly from firefox: No problem, just when called from inside nextcloud.
Using the same machine but chrome/nextcloud instead of firefox, everything works fine.

As m.homeserver is defined in roles/custom/matrix-client-element/templates/config.json.j2, I hope someone here can clarify how to solve the problem.