When I looked through the Update Manager log files (C:\Documents and Settings\All Users\Application Data\VMware\VMware Update Manager\Logs\) I found the following error:

2009-10-05 17:15:41:137 'SingleHostScanTask.SingleHostScanTask{5}' 8032 ERROR vciTaskBase, 577 Task execution has failed: SingleHostScan : Patch Metadata Not Found: PatchID: Unknown[Missing files: http://<hostname>:80/vci/hostupdates/hostupdate/esx/esx-3.5.0/contents.xml.sig]

the problem was that Update Manager tries to connect to the patch datastore with http:://<hostname>:80/ ... I noticed that this was the short name and not the full DNS name of my Update Manager server (replaced here by <hostname> for security reasons). When looking to the dns entries under my ESX configuration, I saw that the DNS suffix of the Update Manager server was not filled in under DNS search domains. I put the search domain in and everything worked smooth from then on.

DNS settings are very important when using Update Manager. In this case the Update Manager server was in a different DNS domain than the ESX servers.

Grtz,

Tom