Two years later & your comment here fixed the issue I was having. Extremely vague “LogSpawn: Warning: SpawnActor failed because no class was specified” with no further details triggering after every Play In Editor run was driving me crazy. Turns out HUD in Maps & Modes had somehow gotten set to None. Not sure how that happened, really.
Wish the editor was more detailed about what was trying to be spawned in that circumstance…