I have noticed that there are problems under OnlyOffice (Nextcloud 21) when at least 2 people edit a document.
This often happens when one person saves and then leaves the document while another person is still looking at the document. If the person who first left the document then wants to open the document again, it is broken and can no longer be opened - even if you download it and then open it.
This is probably because the person who was still in the document prevented saving it.
Is there a solution for this?