In order to fix the issue, you need to perform the following actions:
First of all, please make sure to backup your database before any modifications!
1) Open your Joomla database using PHPMyadmin tool.
2) Find and open (click “Browse” icon) “jos_users” table:
3) Find and open (click “Edit” icon) the record with “Super User” name:
4) Change Super User id value to 42 and click “Go” button:
5) Find and open (click “Browse” icon) “jos_user_usergroup_map” table.
6) Find and open (click “Edit” icon) the record with “group_id” value 8, change “user_id” value to 42, then click “Go” button:
7) Logout from Joomla admin section and login back
That’s all, the issue should be fixed.