Blank video window - video plays fine in clean session


After the 8.1 update Nuendo has become a bit more usable on my PC - I’d switched to Cubase 9 demo for now, but the performance in Nuendo seems to be a tad more stable.

However, when I load up one of my cubase sessions, I get a blank video screen; it’s just plain white.
Re-importing the video, muting it, it all makes no difference.

Importing the same video in a new session plays fine…

What can I do to fix this?