At least 3 users should never have auth set to anything other than manual.
User ID 1: guest (even if you don't allow guest access)
User ID 2: the account established when moodle first installed. Default username if left and not changed was 'admin'.
At least another manual account that is also among site admins.
To see if User ID still exist, query your DB
select id,auth,username,firstname,lastname,email,lastip from mdl_user where auth = 'manual';
If you see a user ID 2 and don't know the password you can change it using command line only script in moodlecode/admin/cli/ called 'reset_password.php'
php reset_password.php will prompt for user name (admin?)
and if found, will prompt for password ... which might have to follow the password settings.
Once changed, login using the standard login form.
Then check one other account that you can set to manual.
When you sync ... exclude user id 2 and that other account you set up for site admin.