There is a bug with World Editor if you save a map, and do ‘test map’ function in a certain order, the map gets corrupted with the ‘Level Data missing or invalid’ error next time you try to open it. It won’t happen right away, but without making a new copy of the map, and repeatedly doing ‘test map’ and save it will eventually happen.
The only way I’ve found to get around is this is if I need to do test map, I will save the map multiple times, then delete the copy that I was using to do ‘test map’, because this is the copy that will likely get corrupted.
I had this issue on a bare bones map with barely anything on it, and no custom assets, so I know it’s an issue with World Editor and not something else. After lots of testing, it seems to be in some ways tied to the ‘Test Map’ function in World Editor, because I can reproduce the issue if I do save and test map many times.
I have three separate computers that I have edited maps on recently, and the issue is the same with all 3.