Same issue here, I solved it by editing the database directly through phpMyAdmin.
Connect to the DB and look for " jos_user_usergroup_map " then change your group ID to that of the super users group. You can view that ID number through USERS > GROUPS, locate the ID to the right for "Super Users". I had to change mine from group ID 7 to group ID 8. I suspect yours will be similar. After that login again to your /administrator back-end and your set.