TL;DR;
The root cause of the issue is " casting the game mode to your custom game mode in a class that’s also referenced by your game mode". (quoting Editor launch stuck at 72% due to a blueprint issue - #2 by Deiain)
SOLUTION
In my case, the issue was related to a KingOfTheHillGameState script that included a cast to KingOfTheHillGameMode. This same GameMode also referenced the same GameState, causing a circular dependency issue.
I identified the problem using Git: I compared the current HEAD with a previous commit (one I knew was working) and reverted the file I suspected was causing the issue.
It turned out that a dangling cast was responsible for the error.