After upgrading from 1.3.1 to 1.5.1 I get this error in the console Unity.Netcode.Editor.CodeGen.NetworkBehaviourILPP: (0,0): error - System.NullReferenceException ...
ArgumentException: An item with the same key has already been added. Key: -1 at System.Collections.Generic.Dictionary2[TKey,TValue].TryInsert (TKey key, TValue value, ...