Two months ago I described a problem with downloading patches for a brand new vCenter 5.1 and VMWare Update Manager installation. Today I faced the same problem upgrading an existing vCenter 5.0 to 5.1 where the SslVerifyDownloadCertificate parameter had already been set to 0:
Download patch definitions
hosting the patch definitions and patches cannot be accessed or have no patch data. Check the Internet connectivity.
Heres' a screenshot of this error message:
Solved (as described in the other post) by setting once again that registry key from 1 to 0.