Ive had errors related to undo group as well while trying to render an audio only file.
I explained the whole fiasco in Todd's post about the new patch being ready. what's new and changed in After Effects CC 2015 (13.6), available now
AE is really broken badly. They screwed up, seems they cannot fix one thing without breaking something else.
I would feel less frustrated if when updates dropped i didn't find that they'd been wasting any time on changing things like panel & menu layouts.
They should be focused 200% on fixing AE's broken core functionality. It's shameful
I'me getting this when copying pretty much any layer it seems - it's really messing me up - I'm pulling my hair out trying to get a project out. Will have to try and go back to an older version - if I can?
What's worse - after this error, if I then paste, AE crashes!
The After Effects CC 2015 (13.6.1) bug-fix update is available. It fixes several bugs, including one regarding Undo.
Let us know how things work after you have installed the update.
Solved my problems. Very stable today so far