Drag&Drop function not working?

The Drag&Drop function has been working fine for the last few days, and I have been able to close and reopen Unity fine without any problem, but now for some reason I can't use it? I searched through my System Preferences and tried re-enabling drag, but it still hasn't worked. I tried unplugging my mouse and using the trackpad, but nothing happened. The error that comes up is: Error in file: /Applications/buildAgent/work/842f9557127e852/Editor/Platform/OSX/DragAndDrop.mm at line 245

It says that no Drag&Drop hasn't been set up? I have no idea what any of it means, sorry. I'm sure this looks a little nooby to some people, but I am only a beginner to Unity and don't really know what is going on at the moment. Thanks in advance :)

EDIT: Copy/Paste doesn't work either? Is that part of the same error or is that a completely new one?