Might be the infamous hang-on-exit problem. That one is caused most likely by plugins, too. There were a few plugins that were built with an older, buggy version of the VSTGUI toolkit.
It might be possible to find out if it is a plugin if you can provide a crash dump of the hanging Cubase process. This is an instruction I found here on the forum:
Edit: I’ve just seen your screenshot and there already seems to be a crashdump. Never seen the “Freeze”-thing myself before, maybe it is new in C12…