After many system instabilities and freezes I was suggested to do a PRAM reset. When I boot now and the File Vault asks for the password to decrypt the SSD the Guest user is gone and my user lost his thumbnail and the name says [Update Needed?].
The solution here isn't working: https://apple.stackexchange.com/a/80122/70851
When I clicking on the question mark in the password box nothing happens. But the worst thing, my password won't get recognized anymore. And I did type it right and I did check the keyboard setting.