I've just downloaded Pureref for the first time, onto my windows ten laptop. (I'm not tech savvy, so I apologize if I'm doing something stupid). I cannot crop images I have dragged into Pureref, when I hold down C I can't drag the mouse to crop anything, I mean the mouse will not move on screen at all, like holding C locks it in place. All the other controls work fine. I've searched all the controls and looked through help but can't see that anyone else is having trouble with this. Perhaps I'm just doing something dumb and it's a simple solution?
Are you using a track pad on a laptop by any chance? Some laptop keyboards lock the track pad when typing, usually it can be configured somewhere but we can't answer exactly where unfortunately. A workaround from PureRef could be to re-bind crop to something else than a single letter key. Modifiers like Ctrl/Shift/Alt usually doesn't lock the keyboard and should work, maybe even if you add a letter key to it as well like Alt+C. You can rebind Crop in Settings->Key bindings->Controls->Crop Selection
Hope this helps!