Unknown error message

Unknown error.  Building basic barn.  2 sheep in area, one naturally spawned pink sheep, the other tagged with "Jeb_" so it's color changing.  Unknown what action causes error to appear.  Building with oak and spruce, don't know if any of it is related.  Picture of error is attached.  Could possibly be another player in family running by who has spyglass, does seem to appear any time you look at the direction of that player.