There is an easy fix to this; login to phpmyadmin, or whatever you want to use to access the database and execute the following query on the db for your piwik install:

INSERT INTO `piwik_user` (`login`,`password`,`alias`,`email`,`token_auth`,`date_registered`) VALUES ('anonymous', '', '', '', '', NOW());

Basically piwik needs to have an anonymous user setup in the DB as it looks for it when you try and access the settings.

-JJ