This post is applicable to customers using VMware vCenter Server Appliance (vCSA) 6.7 Update 3d with vSphere Update Manager.
Problem:
- vCenter Server Appliance 6.7 Update 3d with vSphere Update Manager configured to scan a cluster of ESXi hosts. DNS is fully configured and functioning correctly for ESXi and vCSA.
- The Check Compliance action from the vSphere Client returns the error message: “There are errors during the scan operation. Check the events and log files for details.”
- In the /var/log/vmware/vmware-updatemgr/vum-server/vmware-vum-server-log4cpp.log file of the vCSA instance, there are “HostUpdateDepotManager” ERROR messages with “http://<vcsa url>:9048/vum/repository/hostupdate/vmw/vmw-ESXi-6.7.0-metadata.zip – Name or service not known” listed.
Solution:
- In the vSphere Client, select Home, Update Manager, Settings, Administration Settings, Network Connectivity.
- Change the “Update Manager Patch Store host name or IP address” from the vCSA FQDN to the vCSA IP address from the drop-down menu.
- The Check Compliance action should now complete successfully.
- Try changing back to the vCSA FQDN to verify if Check Compliance still works.
Background:
I do not know why vUM behaves this way even though DNS is correctly configured, but the solution above is a workaround.
vcdx133.com 
I had this issue after upgrading vCenter from 6.0 to 6.7 and was upgrading the hosts. The hosts upgraded OK, I just could not scan them to install updates.
I found that since I was using Update Manager to update all hosts in the cluster in one hit (it actually upgrades one at a time, one-after-the-other), during this time, I could not install updates on the hosts which were already upgraded.
Once the upgrade of all hosts was completed, I could then scan & install updates on the hosts.
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