Found the culprit: logread showed me
block: extroot: UUID mismatch (root: 8f4630a4-71edb3bc-22793c7c-4d016c77, overlay: c9327adf-61e2-1362-8b88-2666395d3da0
Not sure why a sysupgrade would change this, but doing
mount /dev/sda1 /mnt
rm /mnt/etc/.extroot-uuid
umount /mnt
reboot
fixed it for me.