When we get these kinds of errors, we do not have the required information to provide context, as the file is corrupted and relationships between objects in the file are broken.
These errors generally happen when assets are merged, which is not recommended for AnimatorControllers, as they are a complex web of dependent sub-assets.
In order to fix the file, you need to search for the missing PPtr in the text file, and follow the chain of asset dependencies to find what refers to it. Then you can either look at previous versions of the file to identify what was removed, or delete the invalid asset from the text file if it's unused.
In this specific case, there are multiple dangling transitions which do not seem to be used by any states, and which point to inexistent states.
So they should be able to be safely removed.
For reference, here is an example of transition to remove from the file:
--- !u!1101 &5954314337809685864
AnimatorStateTransition:
m_ObjectHideFlags: 1
m_CorrespondingSourceObject: {fileID: 0}
m_PrefabInstance: {fileID: 0}
m_PrefabAsset: {fileID: 0}
m_Name:
m_Conditions:
- m_ConditionMode: 2
m_ConditionEvent: <REDACTED>
m_EventTreshold: 0
m_DstStateMachine: {fileID: 0}
m_DstState: {fileID: -6474349677314147615}
m_Solo: 0
m_Mute: 0
m_IsExit: 0
serializedVersion: 3
m_TransitionDuration: 0.25
m_TransitionOffset: 0
m_ExitTime: 0.75
m_HasExitTime: 1
m_HasFixedDuration: 1
m_InterruptionSource: 0
m_OrderedInterruption: 1
m_CanTransitionToSelf: 1