If for some reason your debian-sys-maint account is unset in MySQL, during MySQL start you will have connect to server error. You might need to set this account in the MySQL server itself.
cat /etc/mysql/debian.cnf to get the password value from the file.
Then connect to the DB
mysql -u root -p
GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY 'YOUR_PASSWORD' WITH GRANT OPTION