Please help with NextCloud Office

Dear Community

I’m getting really annoyed by not being able to run nextcloud office - neither in my hosted installation nor on my synology installation.

When opening a file, I get e.g. “Lade test.odt” (loading) and nothing else.

I run version 24.0.5 on all my installations, using the built-in CODE server.
Under settings - nextcloud office it’s all green.
I don’t see anything special in the logs either.

So, any hints what I can still try to get it running?