So the inventory service is actually up and running.
It still might be worth to check the ds.log for any exceptions.
https://updatemanagerurl:10443 produces what output in the web browser of the system you want install vCenter on? 400 or 404 or something completely different?
Never came across this issue with customers so far but apparently you are not the only one hitting it.
vCenter 5.1 Install fails to validate Inventory service with SSO Admin
Unfortunately no real solution in that thread. So while interpreting the error message could I ask you to provide the logs mentioned by Derek in the other thread as well as the SSOAdminServer.log?
If you are really feeling troubleshooty you might also want to run Process Monitor during the installation and filter for WriteFile events to see if any other logs are touched that could be relevant as well.