Well I just had this issue and in hopes of helping someone else. I just deleted the Engine Generated Folders (Binaries, DerivedDataCache, Saved) and it worked.
Ofcourse if your project is huge and has gigabytes of derived data this means you might need to wait for a while so the engine can re-cache them. But it is better then losing the project or worse: In a hopeless attempt moving/copying the project that big into a fresh one.