With Reaper (and just about only with Reaper) you can be sure that the issue is not the licensing. Have you tried to open it in safe mode?
Open Reaper without the project and go to open it but check the "open in safe mode" or something like that I think. Once it is in safe mode you can turn on one plugin at a time to find what plugin is causing the trouble (if any).
Have a look at this thread
http://forum.cockos.com/showthread.php?t=23297