Nagios Support Forum

Error while connecting to database:
[localhost] Connection to the database server has failed by reason:
Access denied for user 'nagiosql'@'localhost' (using password: YES)

did you get a reply for this error? i am getting the same issue

By default there is no password, unless you set one... so try:

mysql -u root

Otherwise you may need to research a way to do mysql password recovery.