Just wanted to weigh in on this thread as I experienced the exact same problem as accesstime -- and had the same solution.
I've noticed since the last upgrade (I'm now on 2.0.4) that a few odd things are happening, like I often lose all of my windows etc. It might just be that you've changed a shortcut to something that I use all the time (or it could be that Windows updates are processing keystrokes differently. In any case, I'm often having to reset my workspace.
What does this have to do with inpainting? Just that this also feels like it occurred immediately after that last upgrade. My inpainting toolbrush went down to 10% opacity by itself. It certainly wasn't a decision on my part. It's an easy fix but if, like me, you don't touch this setting, it's easy to miss.