I was getting an error where I wasn’t able to download patches one day. This was the fix that I found.
On Windows server where VUM is installed Stop VMware vSphere Update Manager Service.
In Windows Services, stop the vSphere Update Manager Service as below:

Open up Windows ODBC 64-bit, select ‘System DSN’ tab and highlight VMware vSphere Update Manager, click Configure.
Then click Next until you reach Finish:

Click Test Data Source

Came back successfully as below:

If OK, go to the next step but if not look at error and resolve the issue:

Launch the VMware vSphere Update Manager Utility (located Program files/ VMware/ Infrastructure/ Update Manager.) Login with vSphere Credentials.

Available options once you login:

First I would normally try to re-register to vCenter Server. Enter the relevant details and click Apply. Restart the VMware vSphere Update Manager Service.
Try patching the hosts again with VUM. If successful, well done, you are finished.
If you still have issues go to the next step.
Launch VMware vSphere Update Manager Utility (located Program files/ VMware/ Infrastructure/ Update Manager.) and login with vSphere Credentials.
Select Database Settings and click Re-intialize Database, and then click Apply. WARNING: This will reset the database back to default! Any HPE / Dell Custom Baselines and Images you had added to downloads will need to be recreated along with custom baselines.
