This error occur even you had logged in as administrator. Don't panic, it might not due to your daily login username & password incorrect, but may due to some compatibility between BackupExec remote agent with Microsoft Windows.
You may try the following workaround which proposed from Symantec to resolve the issue.
Additional error message as below:
Make sure that the user account has the appropriate permissions and that the password was typed correctly.
Final error category: Security Errors
To correct this, change the service account information as follows:
2. Make sure no jobs are running and close the main Backup Exec window while leaving the Backup Exec Services Manager window open.
3. Click Services credentials.
4. Select the Change Service Account Information check box.
5. Type in all of the correct up-to-date information in the fields so that the credentials match the username that is supplied to the SYSTEM LOGON ACCOUNT.
6. Do not select the Change startup options check box.
7. Click OK.
8. Once the process is complete, choose to Stop and then Start all services in the Backup Exec Services Manager window (select Restart Services in Backup Exec version 10.0 or later).
9. Check the Backup Exec server and the servers with the Backup Exec Remote Agent for Windows Servers to determine the logon account associated with the Backup Exec Remote Agent for Windows Servers service.
10. Make sure that if the Backup Exec Remote Agent for Windows Servers service is using anything other than Local System Account, change it to use the Local System Account so that Backup Exec will function correctly.
*the above info obtain from Symantec Knowledge Base with ID:328478
