Thought I'd snapshot my Lab's vCenter running 5.5 U1 and upgrade the O/S from 2012 to 2012 R2 MU1 which was just released. Don't try this on Production folks!
I took a snapshot and kicked off the upgrade. After the final reboot, all the services started up except the vSphere Web Client. I tried the C# Client but that wouldn't connect either. I turned off the Firewall to see if that helped as the main vCenter service had started and I still got a connection failure. Then I noticed that the IP Address had changed, it was using DHCP. I changed back to the original static address, enabled the Firewall again and rebooted. Now the C# Client worked but the web client service was still not responding.
I did a repair install (remove/install) of the Web Client to see if that would help. This got the Web Client Service into a running state finally and after a few minutes I could connect via the Web Client Interface.
I had the same trouble as this with 5.5.0b and was hoping 5.5 U1 would get around it but obviously not. For Production Servers you have the choice of trying the steps above with good backups in place (!) or build a fresh vCenter on 2012 R2 MU1 and migrate the Database, SSL keys etc.
Note: By MU1 I mean the maintenance update released by Microsoft for Server 2012 R2 in April 2014. There is a set of 6 patches for existing 2012 R2 systems or you can give it the integrated ISO which is the method I used above to upgrade from 2012.
