Possible solution below.
Problem: At least in my case, this was because the editor was not recognizing a .cpp file that was serving as the parent class of a BP. Notably, the .cpp file existed on both Helix Core (Perforce/P4V) as well as my local machine.
Solution:
- Open editor (yields CreateExport warning)
- Open a .cpp file
- Close editor
- Build from the IDE (e.g., in VS Community, Debug → Start Without Debugging)
Should be solved… I also opened the problem level and saved for good measure, but I don’t think that matters.