Stuck at 71% is quite a common issue from my experience. It happens when assets are directly referenced in C++, such as
static ConstructorHelpers::FObjectFinder<UClass> SomeName( TEXT( "Blueprint'/Game/YourProject/SomeFolder/YourBP.YourBP_C'" ) );
Here is a bit more info, but generally removing ConstructorHelpers should fix the issue.This bug is occurring quite randomly, so it probably does nothing to do with the GameMode itself.