I did research on the error, and I’m finding nothing.
Also, all of the steps that you have tried, I would have suggested (like, verbatim; as if you’ve seen my posts or something).
I don’t know what the interface of the UEFN is, but if it’s similar to Unreal’s default editor interface, there should be the option to .zip your projects:
You ABSOLUTELY need to do this for all stable builds.
If I didn’t, I would have had to rebuild entire levels that took months, multiple times.
Your error…
…seems to be triggered by a forbidden pointer/reference that is being accessed by an unauthorized value; you may have deleted something you weren’t supposed to.
Unfortunately, you may need to bite the bullet on this, roll up your sleeves, and start over. I’m not sure why backing up isn’t stressed more in tutorials, but any time you delete anything, make sure that whatever it references is redirected accordingly.
I could be off, but until I learn more about what the error means, it’s all nebulous to me.
Good luck.
PS: I don’t think this will help, but just to cover your bases, you can try Fixing Up Redirectors (if this option is available in your version of the Editor):
Do this for EVERY major folder.