vCenter Appliance 5.1 - Cannot verify the SSL Thumbprint

Hi,

i have installed a vsphere environment with two vsphere ESXi 5.1 hosts and a vCenter Server Appliance 5.1

Recently a strange issue has shown up. Randomly both hosts became disconnected from vCenter with the error "Cannot verify the SSL Thumbprint".

To resolve this issue i have to login into vCenter and re-verify both SSL Thumbprints and reconfigure HA, but after a random time everything happens again.

The thumbprints of both server from DCUI match the ones in vCenter.

I updated to the latest version of vCenter Appliance, 5.1 Update C, but the issue is still there.

Anyone as any idea?

Thanks a lot for your help