Multi Node Upgrade
I upgraded our Zimbra multi node setup from 5.x to 6.x but after the three nodes had all upgraded webmail complained about nodes having networking error. I never did find out what caused the problem. (I even tried redoing the certificates.) Eventually I had to recover from backups. I'm wondering if the problem is related to with our OpenVZ setup but I have successfully upgraded OpenVZ single node setups so I kind of doubt it.
Welcome to the forums!
If you can post some specific errors from the logs, that would enable us to help you better.
All the best,