I came across this error recently when doing a POC for XenDesktop on Vmware ESXi 5.
Error: Unable to contact the hypervisor. Check that the address, user name, and password are correct; the Controller and hypervisor are connected; and the SSL certificates have been set up correctly if you are using HTTPS.
To get around this issue I had to change the Proxy.xml file located on the Virtual Center to accept HTTP communication which is located in the following directory
Find the following sections of the XML file and change them from "httpWithRedirect" to "httpAndHttps"