Infinite loading screen

I tried to switch from OnlyOffice to Collabora, as the community document server of OnlyOffice has sooo many bugs.
I installed the Nextcloud Office app and the Collabora Online - Built-in CODE Server app. In the nextcloud settings page it shows that the server can be reached. But when trying to open a document, I´m getting an infinite loading screen and no error message.

Screenshot 2022-10-05 at 11-21-45 protokoll_2022-09-20.odt - Region im Wandel

Any ideas where I could even start looking for the problem?

error_log_apache tells this

[Wed Oct 05 11:29:10 2022] [error] [pid 23427] mod_proxy_fcgi.c(1006): (70007)The timeout specified has expired: [client] AH01075: Error dispatching request to : (polling)
1 Like