Hi guys - although this is an old thread I just wanted to highlight that I came across this same issue when performing the latest updates, so it is still a thing it would seem.
Update Details:
Processing triggers for rsyslog (8.2102.0-2+deb11u1) ...
Processing triggers for openmediavault (6.9.4-2) ...
Fix:
chmod o+r /var/lib/php/sessions
Thanks for the advice above which got me up and running.