Thank you for submitting this support query. Please follow the steps given below to isolate the issue:
1. Enable Restrict Anonymous support on both SQL server and backup server.
http://support.veritas.com/docs/238618
Although, enabling the above option helps in troubleshooting Exchange issue, it has been seen that this is helpful in resolving the communication issues.
2. Create a new account on the server which holds the SQL database. Make this account a member of Local Administrator group. Also, make this account a member of Domain admin group. Provide this account with DB owner rights on the SQL server.
3. Create a new Backup Exec logon account with the same username and password.
4. Perform a backup and observe the results.
Hope, this should resolve the issue.