Wish I had an answer for you…
I’ve been running into this too very recently, in 5.3.2. First time it happened I was able to roll back to a couple hours prior and everything was fine. Recreated my work without issue and it was fine thereafter.
The next day it was still good, but again after a couple hours of widget work, I ran into the same problem - tried to package the game and hit build errors. Rebooted the editor and hit the same “Failed to load Outer” spam. I haven’t done anything fishy or used Live Coding.