So I’ve recently had this same error happen in our project, and even if I go to a backup and pull it and try and use it, or re-create the map, the error has come up twice now, on two different versions of the same map.
I am using Save-As, saving an existing map as a new map, can save, and edit the map blueprint, but at some point when I save, exit unreal, and come back, I get the “map was saved using a newer version of Unreal” error.