Just chiming in with my experience, which I posted in a separate thread. I was having very similar issues, and seem to have found a fix for my system, which was to toggle the new option in Preference, Click in top half of ruler to activate loop.
I managed 5 hours of uninterrupted work after doing this. Not sure if this was a fluke, or a working ‘fix’. Previously I couldn’t work for more than a few minutes before Cubase locked up as described by many here. Will continue with 10,5 today and see how things roll.