% shutdown -c
...didn't work as it couldn't identify the process id to cancel. Logging out and in again a few minutes later revealed the same 2min message. Unsure what to do I read the shutdown man page which revealed that during the last 5min of scheduled shutdown, logins are prevented. This explained the symptoms. I searched the process list but there was no trace of the shutdown utility running. Googling the symptoms revealed nothing so I tried scheduling a(nother) restart (from within root terminal) with a delay of 0min to see if this would clean up / overwrite any left over/corrupt system files:
% shutdown -r now
Fortunately, this worked. I couldn't find evidence of anyone else encountering this problem, so I figured I'd note down the solution here. Essentially I've learned that if you set up a scheduled shutdown, its safest to cancel it explicitly using the shutdown -c command, instead of hoping it will cancel itself when you pull the power.