I’m having (what I think is) exactly the same issue, but on the Mac build of 5.6. The blueprint can be opened until an editor restart, at which point UE declares it to be invalid. The blueprint never appears in the list of options for Default Game Mode, even immediately after creation, when the blueprint seems to be functioning and editable.
Mac is different in a couple of ways. I’m using Xcode as the external development system. Mac doesn’t support Live Coding, so apparently the UI regresses to an older system called Hot Reload. I still get a Compile button on the lower toolbar once a C++ class is added, but there’s no meatball menu to turn off Live Coding.
But, I’m pretty sure this problem doesn’t have anything to do with live or hot coding. The project builds fine in Xcode whether UE is running or not. If I build with UE not running and then restart the editor, the complaint is exactly the same: invalid blueprint.
I’m a bit concerned that what is literally the first hands-on tutorial in the documentation seems to be completely broken. This doesn’t bode well. If I spoke French, I’d say “C’est de la daube, laisse tomber!”