We got the same problem after upgrading our vCenter Server to 5.5 U3 running on a physical standalone windows 2012 R2 machine,
turns out the problem was that the web client was installed on our D-drive - it worked on 5.5 original/5.5 U1/5.5 U2 on the D-drive without any problems funnily enough...
We just uninstalled the web client and did a reinstall on the default C-drive and path and everything works after that. Weird though...