Access denied Error in MySQL and Drupal
May be lame to do this but worthy to double check the username and password and ip. "Copy paste the exact connect string in commandline"
Triage more by only keeping DB connection from app/web server and check the DB connectivity  functionality and see if the issue repeats. If yes, do the same from someother host with same app/web interface server version. [with this we can be sure that the host is no problem] .