I've encountered this issue in a big way, have done a bunch of poking around to figure out what's causing it, and reported my findings to the Props. Check out this tread for more details (when I started the thread, I thought Waves plugins were the problem, but I've since discovered that's not true):
viewtopic.php?f=4&t=7510557
What I've found is that Reason starts displaying corrupted thumbnails, along with other graphic glitches, when the installed plugin count (VSTs + REs) exceeds a certain number, which seems to be around 1000, based on what I'm seeing on my system. And actually, it's not the amount of plugins that's critical, but the amount of thumbnails that Reason has to manage.
So until the Props fix this, here's what can be done to resolve this:
- Try taking the snapshot again. Usually this will only fix the problem for the current session. The next time you launch Reason, the thumbnail glitch is likely to appear again.
- Remove some thumbnails. Reason stores thumbnails in the user directory on Windows (I don't have the exact path in front of me right now, but just dig around and you'll find it). Pull some thumbnails out of that folder and the problem should go away. But of course, some of your plugins won't have thumbnails.
- Remove some VSTs. I got rid of a few VSTs that I never use, and the glitches went away.
The above info is by no means "official", it's just what I've observed on my system after trying a bunch of stuff, so there could be more to it. But this is what I know so far.
Hope that helps somewhat.