When I crop (within the develop module) in Lightroom 4.1 RC2, (or in 4.0), after a few photos, it simply stops working. When I try to crop I either get a black screen where the picture would normally be or get an "An Unknown Error Occurred" message. Getting out of LR and back it solves the issue for a few more photos, but then I have to keep doing it. I did not have this issue in LR 3. I can't seem to find others with this issue.