Site stopped working overnight - 'not writeable' error

I'm running 2.2.5 and yesterday, I had a site that was working just fine.

This morning, the admin page I left open in my browser was prompting me to sign in again. And when I tried, I got the 'Grav Problems' page telling me there are problems with 8 required folders: cache, logs, tmp, backup, images, assets user/accounts and user/data

All are declared 'exists but not writeable'

The webserver is Apache, with php 8.5, running under user/group www-data. All the directories have 775 permissions.

And yet, I just can't get this error to go away. I'm absolutely mystified as to what could have happened overnight - the only thing that runs from cron is a task that backs up to rsync.net.

Nigel.

Please don't post your question in more than one forum (cross-posting). In each forum, people will be spending time and effort to answer the question and in the end, the time and effort in all but one will be wasted.

Instead, choose the best channel and wait a few days. Then post in another place. Provide links between each forum and repost the answer (link) in each location.