Can't login in admin anymore

Hello,

Today all of a sudden I can't login to my admin section anymore. It says "Corrupted source code."

When I login through SSH and view the Plesk panel.log it says:

[03-Jun-2016 13:55:17 Europe/Berlin] PHP Warning: hash() expects parameter 2 to be string, array given in /usr/local/psa/admin/plib/auth.php on line 158

What is causing this error, and how do I solve this?