Received this error when I was working with my Hyper-V servers. It's easy to solve but the hos machine has to be rebooted and therefore all the VM machines will be unavailable for a while during the reboot.
Cannot connect to the virtual machine because the authentication certificate is expired or invalid. Would you like to try connecting again?
This error is due to the VMMS certificate expiring. So just run the KB fix and the issue will be gone!
More info: http://support.microsoft.com/kb/967902
Direct download of the fix: http://www.microsoft.com/downloads/details.aspx?FamilyID=289c831c-9142-4c00-bacd-6c1924ff0ecc