This sounds like a missing file, does the file mojoEncryption.config exist in the root of the web? If not then upload it.
Password format is set to hashed by default, if all else fails you could get past this error by changing to clear text. To do that you would have to go directly into the db in the mp_Sites table set PasswordFormat = 0
Then in mp_Users overwrite the hashed password with a plain text one of your choosing.
Then touch Web.config to recycle the app or if your control panel has a button to recycle use it.
Hope it helps,