Sucessfully updated to 5.1 U1c.
5.1 U2 SSO installer is still complaining about wrong or blank password after i renamed/moved the JRE folder. The password check itself and warning dialog box is displaying unusually fast which leads me to conclusion the installer actually uses JRE in "Program files\vmware\infraestructure" for some reason, because when i do not rename/move the folder then the password check takes a few seconds to complete.
So now i am stuck at the same point. I will try to upgrade from 5.1 U1c to 5.1 U2 later this week as i need vCenter up now for Veeam backups to work, but i fear the update will mess things up again and i will have to do full restore again.