Stefan, I got the exact same error when manually upgrading my DokuWiki installation (“Anteater”).
I'm was surprised that I wasn't able to find any documentation about this, since apparently it looks like a bug was introduced.
The problem lies on PHP not being able to find a PassHash Class.
I made a recursive grep search and indeed the class PassHash was completely missing in action.
Here is what I did to fix this:
I manually created the PassHass class in inc/PassHash.class.php with http://xref.dokuwiki.org/reference/dokuwiki/inc/PassHash.class.php.source.txt
and updated inc/auth.php to also require the PassHash.class.php class in the auth_setup() function.
Then you'll need to reset your password using the DokuWiki forget password feature.
I hope this helps.