Some people over look this when upgrading to vCenter 4, even though it warns you before installing vCenter… yeah I fell for it also.
Upgrading VirtualCenter 2.5 to vCenter Server 4.0 Update 1 without upgrading VUM and Converter might result in certain issues
If you upgrade VirtualCenter 2.5 to vCenter 4.0 Update 1 without upgrading VMware Update Manager (VUM) and VMware Converter, you might encounter the following issues:
When you are logging in to vCenter Server, the vSphere Client might display the error message “The request failed due to an SSL error”. Clicking OK for this message might display the error message The server could not interpret the client’s request. Login might succeed if you click OK for this error message.
So easy fix, as it says above, you haven’t updated VMware Update Manager, go back to the vSphere vCenter CD and run the Update Manager Installer, the rest is easy from there.