Since Unity 6.3, we have observed an issue with the Scene Templates (such as the FPS Player and Sky View Player). When a scene is created using one of these templates, an error is thrown, resulting in broken references (specifically within the Game Manager for the 'player' and 'ground' links).
Assertion failed on expression: '!(transfer.IsRemapPPtrTransfer() && SerializeTraits::MightContainPPtr() && transfer.IsReadingPPtr())'
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)
The current workaround is to save the new scene, open a different scene, and then immediately reopen the scene that was just created; the scene will then load normally without any errors.