Have you checked both /Data/Persistent/Encryptionkey and Data/Persistent/Cache/Data/Flow_Security_Cryptography_HashService/encryptionKey?
Don’t have any other idea what could be wrong.
Had Same Problem, - even on every login by Setup, - the Setup password was refreshed immediately and blocked my login constantly.
Finally, the problem was resolved after updating the Neos.Setup package with “@dev” … (instead of “… dev”)
For me the problem seems to be maybe in relation to the Setup Package … ?