Wow, I never would have even though about checking GameplayTags for the cause of the error. But it makes sense. Having a blank level with a simple ground plane works fine, but as soon as I add an actor that has a GameplayTag Container, I get the error.
Does this mean I should go through everything and remove the GameplayTags and find another way to get the game to work? Or should I continue as is and hope for a bugfix?
PS. I upvoted your bug report. 