I’ve got this error everytime I try to create a group with the setting “Allow users to send membership requests to group owners”.
When you create a new custom group you can’t add the owners immediately because you need to save the group first.
Probably this particular setting need some review. At least it can be removed here, you can see it after you save the group
I don’t create groups frequently so I don’t know if this is an intentional behavior but, to me, it’s something strange.