When upgrading vSphere 5.5u1 to 5.5u2 I upgraded as recommended by the installer and didn;t have any problems until I ran the Update Manager Installer from the splash screen. It detected correctly it was an upgrade but It didnt accept a valid vc username & password on the step to connect to vCenter using a IP/FQDN, Port Number, username and password. I was able to login to web client using the same details. This is on a patched Win 2008 R2 standard 64bit
Workaround for me
Cancel the Update Manager installer and navigate to the installation folder updateManager directly on your source iso/folder and run the Vmware-UpdateManager as administrator.