I have the same problem, with the same error message. I’m on Win10 Home 64 bit 21H2, Cubase Pro 12.0.52, Groove Agent SE 55.1.11.176. Groove Agent SE 5 is the only active plugin. The problem occurs when editing, I’m not trying to playback via MIDI at the same time.
I submitted several crash dump files in another thread here:
https://forums.steinberg.net/t/cubase-12-pro-still-crashing-on-exit/773320/48
This was when I encountered the ‘freeze on exit’ problem in Cubase Pro 12.0.20. I mention it because I was then told in that thread that Groove Agent SE was causing the instability problem, but I never heard anything further. As I reported in that thread, I also encountered the issue in Cubase Pro 11.