(15:48:11) | PlayerState was from old protocol or different seed, or not from a loaded save. Clearing player state

(15:49:38) | Server Exception: Player Update

(15:49:38) | AssertionException: connection != null

Assertion failure. Value was Null

Expected: Value was not Null 

After the server froze, I restarted it and the above situation occurred. Please help me!