No login possible into the Joomla Backend after using the Quickstart Package
Navigated to phpMyAdmin and select the database for the Joomla! site in the left-hand drop-down list box. Click on my database on the left side of the screen.
Press the "SQL" button in the toolbar to run an SQL query on the selected database. This will display a field called "Run SQL query/queries on database <your database>".
Found two additional super users and delete them!
Then you're able to login