Your JFFS2-partition seems full and overlayfs is mounted read-only.
I'm getting "Your JFFS2-partition seems full and overlayfs is mounted read-only". errors.
I can't seem to delete anything from the /overlay/upper/... directory
I have not rebooted yet as it is controlling my garden watering and I'm away atm.
Just an update, I did a reset to default to fix the problem. Not ideal but I got no where with other options.
First step in debugging would probably be to explore what was ending up in the overlay.
One likely culprit is upgrading large amounts of system software (python, etc) or installing many new packages.
The other is having some ongoing task that creates log files or data objects