PUT and DELETE give 500 errors

The frontend seems to be ok - I can login to the webinterface with no problems and the desktop app gives a tick in the desktop, however if you look more closely nothing is updated and all PUT and DELETES give 500 errors. In terms of logs, all that I can see are errors in the Apache logs such as this:

xxx.access.log:192.168.1.161 - njh [17/Sep/2022:13:47:48 -0400] “PUT /nextcloud/remote.php/dav/files/njh/Documents/Owncloud/afile.txt HTTP/1.1” 500 1806 “-” “Mozilla/5.0 (Macintosh) mirall/3.3.6git (build 7956) (Nextcloud, osx-20.6.0 ClientArchitecture: x86_64 OsArchitecture: x86_64)”

Where do I start to debug this? It used to work fine and now it doesn’t and I have no idea where to start.