It's happening with me too. Got a GTX 960, it minimalizes and I can't maximize it again, but the audio continues running well. If someone fing a solution, you're welcome. Happened 3 times in the last 15 min of gameplay.. unplayable to me. And when it happens, the only way to start playing it again is killing the process and restarting it all over.
The lockup in the inventoty happened once.
I don't know if this guy is having the exact same problem, but found this
The problem is not with the Gog Galaxy, game minimized while I was running it on Steam as well.
Sorry for bad english and good luck.