@nebulon Thanks for your reply and sorry for the delay - I do still have the same issue.
I avoided the error by using tarballs to backup rather than rsync. However, I now want to sort it out properly with rsync and hard links as the space taken by tarballs is just too much.
I ran the backup again yesterday and I still have the same error message.
To answer your questions:
I have remounted and retriggered the backup on several occasions - it's always the same error.
I am using encryption, yes.
It does always seem to be the same application that causes the problem. Looking at the ID and browsing the /home/yellowtent directory on my server, I think the UUID refers to my Nextcloud installation. Admittedly, that would be the app with the most data (by far).
Perhaps I could try running the backup with Nextcloud offline to see if that works. It wouldn't be a permanent solution, but it would be interesting to know if that works at least.
Also, perhaps I should be setting up a separate "volume" for my Nextcloud data and migrate it over. I am a little nervous about that though because I'd have to make sure that all the sharing links etc in Nextcloud still worked.
Any other ideas how to troubleshoot this? Thanks in advance.