Okay, whew. Everything has been fixed. Urgh. What a mess. Here are the steps I had to take:
In World Settings, change GameMode Override to BP_SpapDemoGameMode.
Changed the Default Pawn Class to ThirdPersonCharacter.
The above is needed to actually build the level, since it has to be built at runtime.
This almost solved all the problems. However, I didn’t have the Dungeon actor at location 0,0,0. Moving the Dungeon actor to this position made everything work as intended.
Thanks to everyone that tried to help!