This solved the problem. At least I think so, I haven´t had a single error lik this in 48hrs now, and it usually shows up several times a day.
In my Nextcloud jail I pointed collabora.example.com to the IP of my docker host running Collabora. That didn´t work, since Nextcloud is calling Collabora on port 443, and the container is listening on 9980 only. So instead I pointed it to my reverse proxy, and since then the error seems to have stopped happening. Thank you for pointing me in the right direction.
I did a trace before the change, and saw that requests went all the way up to the Internet and back, so pointing directly to the proxy was of course much faster.