Just out of blue started receiving the following error after trying to scan my ESXi hosts with baseline group:

“The host returns esxup error code:99. An unhandled exception was encountered. Check the update manager log files and esxupdate log files for more details”

well the /var/log/esxupdate.log on ESXi host showed the following error, but not much else:
Metadata.pyc: INFO: Unrecognized file vendor-index.xml in Metadata file

Resolution:

Tracked down the KB: 2043170 which allows for the update manager database to be rebuild without having to reinstall update manager:  Please be aware that you will loose all your settings, bundles, drives, extensions, etc! 

1. Backup the update manager database
2. Stop update manager service
3. open command prompt on server where update manager is installed
4. Go to update manager installation folder
5. run the following command:
1. “vciInstallUtils.exe -O dbcreate -C . -L .”  (Both .  should be used) 
6. After completion restart the update manager service
7. Login to vcenter server 
8. Within update manager, within configuration tab perform the “download now” task.
9. Re-import any offline bundles, async drivers, third party extensions, and ESXi upgrade bundles.