The locked door near the end of 9-2 is broken.

The knight's key opens it, but there's an invisible field you can't walk through (not supposed to be like that). I can see the riddle demon, but crossing the threshold is impossible.

Anybody else encountered this and found a fix?

I think I'm going to have to ask for a refund, too many frustrating bugs.

Apparently this is part of the "chess puzzle" bug. It's been mentioned quite a bit over on the Steam forums and supposedly the developer has been working on a patch for several months. I was not able to go through the doorway either. Unfortunately if you unlock the door, then save and exit, the door will lock again but you will have already used up the knight key. I was able to bypass this since I had the goblin in my party but if you unlock/save/quit and don't have him in the party you're permanently SOL.

I have the same problem. once used the knight key can be gotten back at the same location
it was first found - so I can reopen the door every time - but still can't make it through the wide opened door.

So what was behind the door anyway?

I can't be arsed to start all over again and find out. Especially given the continued bugs apparently still present.