Reported internally

Repro steps:
Load attached project
Add SerializeReferenceEnumCrash MonoBehaviour to a GameObject

Result:
Crash

The crash occurs in GetFieldIdentifierForEnum(),as the code tries to determine the path for a field to see if it is an enum, in a very specific scenario.
This only occurs when the Class containing the enum field is embedded (by-value) inside another Class of exactly the same name, but with a different namespace.

A workaround is to use [SerializeReference] instead of by-value serialization for the inner class, because Unity will be track it independently of the enclosing type.