This looks like something that would be in your Anim Instance, your Anim BP asset, are you using sync groups?
UE4 Sync Groups
Does this ensure happen every time you restart editor / run PIE?
Does it happen if no characters are in the level and you just use the DefaultPawn?
“SyncGroup with GroupIndex=1 had a non -1 group leader index of 0 in asset CharacterMesh0”
Solution may be as simple as changing your sync group in whatever Anim BP is triggering this ensure.
Rama