I had an odd problem with a new empty project.
I created a folder with parentheses, I thought it would have given me an error but it allowed me to create the folder. Once created I made a Game Mode blueprint. When I tried to set that game mode, in world settings, I could see the name of the blueprint, but it would not allow me to set that blueprint.
It was very frustrating when I clicked on the correct blueprint name, but the selection stayed as None.
It did have three errors in the output log, but this seemed like an odd error to me for what I had done: LogPackageName: Error: DoesPackageExist: DoesPackageExist FAILED: '/Game/' is not a standard unreal filename or a long path name. Reason: Path may not end with a '/'
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