FIXED!
Attempt these at your own risk, but it fixed this issue for me.
- Exit UEFN and Epic Games Launcher.
- Browse to: C:\Users\YOUR-USERNAME\AppData\Local\Temp\ .net
- Backup the “urc” folder to another location
- Delete the “urc” original folder
- Relaunch UEFN and everything working again.
I found the issue as everything pointed towards Unreal Revision Control being the issue, so I found the urc.exe file in C:\Program Files\Epic Games\Fortnite\FortniteGame\Binaries\Win64\
I ran that urc.exe to see what would happen and got the following message:
From reading that it sounded like something was corrupt in that urc folder, so deleting it and letting UEFN recreate it seemed to work.