More stuff I've tried:
I read the latest update log and noticed it involved some session-related stuff. Since the latest update.tar.gz was dated oct 15th (before the new version was released) I thought I'd give the manual update (as in the knowledge base) a try, but to no avail. I also tried manually creating the new session dir and chmod/chown'ing it according to the instructions in the version history, and also checked directadmin.conf to see if the session_dir directive was set (it wasn't, and should hence default to the new session location).