The error is caused by the default task credentials get lost after upgrade. The default task credentials are stored in a file QMSSystemConfig2.dat. During an upgrade to 7.3 from any previous version it involves a rebranding from Quest Software to Dell. The QMSSystemConfig2.dat file is supposed to be copied to the new installation folder under C:\Program Files (x86)\Dell\MessageStats. If you manually uninstall the old version before upgrade to 7.3, this step would not be executed and the default task credentials will be lost.
RESOLUTION 1
1. To avoid setting the credential explicitly each time, navigate to the previous installation folder, (C:\Program Files (x86)\Quest Software\MessageStats) and locate the file named "QMSSystemConfig2.dat".
2. Open services.msc and stop the Dell MessageStats Scheduler Service
3. Backup QMSSystemConfig2.dat in (C:\Program Files (x86)\Dell\MessageStats) and replace it with the file in the previous version found in Step 1.
4. Start Dell MessageStats Scheduler Service.
RESOLUTION 2
1. Edit the Task that is having the issue and navigate to the Task Credentials page and unselect "Use Default Configuration" and manually type in the Task Credentials.
2. If the task now works that can indicate a problem with the Default task credentials in MessageStats and the passwords should be updated by following this article:
https://support.quest.com/messagestats/kb/7401