Sounds like that hard drive is a bit cactus…
OK the Program Files folder probably has security issues with the constant reading and writing, thats why lots of programs put game save files somewhere in your User folder.
Mak a snapshot now with check-in changes, by clicking on it in bottom right of screen and write launch error or something descriptive in the changelog
Close UEFN and move your project folder to somwhere else like C:\UEFN\
Open UEFN and see what that does