Solution: edit /etc/fstab and remove the mount options acl and user_xattr, if present. Both where in my case. Clear cookies and offline data for website (firefox issue was cleared by doing that)
In reply to @dnutan in another thread that seems similar but unrelated, he asked me to post my fstab, after my journalctl revealed the first error to be a mount option (acl):
…removed the obvious part from that file;
…removed the other (now) also obvious part (requiring reboot to previous kernel), and all seems well.
Need to see what this does for shares and the like.