I just got hit with this issue tonight. Are you using unreal revision control? Did you have anyone join or sync your project around the time this error occurred?
In terms of nailing down a potential cause or reproducibility here…
Between my last successful launch of gameplay and the first time I encountered this issue, I did a few things:
I had someone else sync the project and try to launch it
I had migrated assets from an older engine version (4.27 I believe)
I may have deleted a few assets
I think at this time I tried converting a skeletal mesh into a static mesh but I can’t recall when I did this
My next step will be trying to remove some of the recently added items from UE 4.27 to see if that is the culprit.
- edit -
Helpful to note that content may not pass an automated moderator - likely an asset.
The likely asset for me is a military vehicle that may or may not have something in the material that fails validation.