There are some answers on SOF but in my case this was caused by no more disk space.
Checked permissions and they had the web server user www-data with write access to all app/tmp/*
Checked the /var/log/apache2/error.log and found a “no space left on the device warning”
Therefore did a disk clean up and suddenly started working again
Just toasted my Windows 10 Pro install with a Windows 11 upgrade. Think it will be unrecoverable (because of Bitlocker...