I got this from upgrading to 2021.2 from .1
It happens only when opening the playmaker editor window. Builds are working fine.
TypeLoadException: Could not resolve type with token 010000c3 from typeref (expected class 'UnityEditor.Experimental.SceneManagement.PrefabStage' in assembly 'UnityEditor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null')
HutongGames.PlayMakerEditor.FsmEditor.OnEnable () (at C:/Projects/Playmaker_1.9.1/Projects/Playmaker.source.unity/Assets/PlayMaker/Editor/Classes/FsmEditor.cs:558)