Does the map load correctly when running the game? I’m having a similar problem at the moment with some of our umap files in the editor, but the game opens them fine. I don’t get the “xxx.umap appears to be an asset file!”, but assets do appear in the asset viewer instead of the umap.
Here’s my thread on my problem
I know this isn’t really an answer, but it does seem like a related problem.
Cheers,