AppArmor complaints would be shown in journalctl too. But dmseg doesn't show anything either. Just switched dovecot back to these log files, waited for the error message, yet dmesg doesn't have anything new since yesterday.
Systemd was also my guess as it was originally set to ProtectSystem=full for dovecot.service, but reducing that to yes didn't change anything.
These are the in the Service block of the service, maybe you see anything that could be a reason too:
[Service]
Type=notify
ExecStart=/usr/sbin/dovecot -F
ExecReload=/usr/bin/doveadm reload
ExecStop=/usr/bin/doveadm stop
PrivateTmp=true
NonBlocking=yes
ProtectSystem=yes
ProtectHome=no
PrivateDevices=true
Restart=on-failure
Best
Richard