Really looks like a firewall/proxy issue. Our collector requires 443/https port to be opened to access vSphere API, if you have a web-proxy in your infrastructure it can be applied as a default configuration for the server and all local URLs may also go via proxy which will not work in most cases. Make sure you disable proxy for local addresses, you can even add address of the vCenter server in the exclusions list in the Internet settings or disable proxy in the internet settings completely if the server doesn't need to access the Internet.
I'm sure that our Support Team can help you resolve all such issues, we have really professional and experienced people there. But feel free to post here if you have any other questions or thoughts. We woudl appreciate if you provide us with solution of your support case.