We are using Collabora Online and we ran into the "Error in writing sub-document content.xml" error quite often when trying to save an odt document. We've already filed an issue with them and they confirmed that this error is produced by the LibreOffice core (see https://github.com/CollaboraOnline/online/issues/3457).

We are running the collabora/code:6.4.13.3 docker image which ships with LibreOffice core commit 70f6522 (6.4-49) according to the help dialog.

Unfortunately we tried a lot of things but didn't find a reliable way to reproduce it. We also tried different documents from our LMS, locally generated and even sample files from the internet and they all have a chance to run into this issue. We tested editing the same file with multiple people and while some could save it, for others nothing happened and the error appeared in the logs.

The error isn't saying too much about what exactly went wrong so I'm not sure what additional information I could provide that are helpful. If there is anything I can provide please let me know.