Im struggling to get web client to work on my 2012 DC test box even though I installed Desktop Experience.
I also get errors with Chrome and Firefox. A step backwards IMO.
It works on a client OS, but I only have access to the server remotely in my test environment.
I used to run vsphere client 5.1 so being prevented from running 5.5 was also bad.
If you are in the same position and need to install it on a DC just fire up SuperOrca or Orca and change the LaunchConditions to the following text
( Installed Or (VersionNT > 501) Or (VersionNT = 501 And ServicePackLevel >= 2) Or SKIP_OS_CHECKS="1" )
The client then installs, executes and works fine.