BE2012 unable to backup ESXi5.1 VM using root credentials

Hi experts/guru,

This issues is very frustrating to me. I've successfully created backup job to backup ESXi host 5.1 virtual machines using root credentials and the backup has been running fine for weeks and no changes has been done since then. But one fine day, the backup failed with below error.

"Check status : Error: e0009b61, The job failed with the following error: Backup Exec could not log on to the server with the logon account specified for it. The logon account does not have valid credentials. Ensure that the user name and password are correct in the logon account."

I managed to test credentials successfully but test run was failed.

Please help!!

Thanks