Hi,
I am assuming that the version of Multi-Hypervisor Manager that you are using is 1.0 and the version of vCenter Server is 5.1. Please, let me know if that's not the case.
Is by any chance your Multi-Hypervisor Manager server being identified with Fully Qualified Domain Name that contains uppercase letters (i.e. vcenter.MyDomain.com)?
P. S. I saw you also made a post in another thread (for v. 1.1 Beta) which discusses this issue, so please let me know if that was indeed the issue you were facing and if you were able to resolve it.