By luck I tried to see past UE4 release threads… and man I glad I found one post mentioning the same error message in https://forums.unrealengine.com/showthread.php?82266-Unreal-Engine-4-9-Released!&p=381748&viewfull=1#post381748. It is basically FBX animation. So I guess I should just re import the offending FBXes and be done with it. Hopefully.