Meaning of Error: Unknown managed type referenced: [Assembly-CSharp]

After every reload i get the error message:

Unknown managed type referenced: [Assembly-CSharp] AI.BehaviorTree.Nodes.Transition

I can't find Transition but I do have a TransitionNode.

There is no Transition file in my assets and searching in visual studio doesn't show anything.

I have tried looking online but most search result I find is about [SerializeReference] but i fail to understand how it's related.

 

What likely happened is that you renamed Transition to TransitionNode which caused your old references to break. And because Unity... messed up, SerializeReference never recovers from that.

https://forum.unity.com/threads/ser...en-class-name-is-changed.736874/#post-6557317

This post tells you how to rename a class and keep your old references intact, and if you scroll down a bit further I have a post on how you can make sure this problem never happens again (if you forget to add [MovedFrom] or want to actually delete the script instead).