I'm using LR6 and seem to be having an issue with cropping. When I enter Develop mode, and select crop, it allows me to crop once. But if I try to reselect it, all I get is a blue screen with white lines running from corner to corner. If I deselect the image, return to Library Mode, reselect the image. Then I can continue to adjust the crop of the image. Any advice would be great.
Turn off graphics card acceleration. This is a bug in the GPU acceleration in LR 6. Go to preferences->Performance and uncheck the option you see there. That should fix it.