cannot login to vcenter server 5.1 from vsphere client 5.5

Hi

I cannot login to vcenter 5.1 version from vsphere client 5.5, There is no webclient configured in this environment. I am trying from my laptop client machine.

i can login to vcenter server from inside server ( through vsphere client installed in vcenter server).

One more thing, we have separate vcenter 5.5 environment and i am successfully able to login from my laptop. this 5.1 Vcenter we recently acquired and started supporting. My user account is mapped to administrator role in vsphere. Appreciate if any one can assist.

Thanks

Muhammad