Adding the host into Vcenter getting error

Hello All,

I have added one Host which is ESXi5 into the Vcenter and while trying to add another host ESXi 5.1 getting error and i am trying to place these two hosts into a cluster. Can someone please help me why i am getting this error?

The Recent Task shows as in progress while the host is getting add but after some time gives error with the error message" A general system error occured : Timed waiting for vpxa to start"

I have even tried to add the host from the Power cli but same.

Please do the needful in sorting this issue, is it a compatability issue of placing the ESXi5 and ESXi 5.1 in the same cluster ?

Thanks

Arvin