We haven't migrated to 6.x yet, but looking at the message doesn't make me think it is a problem with the certificate at all. I could be wrong.
a) What makes you believe the cert needs to be regenerated? Is there a log entry with cert errors?
b) Why don't you have access to the admin GUI? You can reset the admin password from CLI and you can use ssh to forward ports through a firewall if necessary.
c) I hope you've already tried to restart Zimbra.
sudo /etc/init.d/zimbra restart
I haven't run SuSE in years, so that cmd could be off a little.
Whenever I see Zimbra connection errors, I have to think that it is a /etc/hosts or DNS issue first. Does the Zimbra server have access to it self using the FQN that Zimbra knows as itself?
To get better help, you'll need to attach the interesting parts from few logs from /opt/zimbra/log/ and /var/log/zimbra.log.
Lastly, I think the certificate handling in 6.x changed from what 5.x does. Did you read the wiki article on 6.x and certs? Here's a 5.x
Problem with Certificate can cause MTA Failure - Zimbra :: Wiki article. Here's a forum thread on 6.x certs
[SOLVED] Certificate problem with 6.0.5
I hope these questions don't send you in the wrong directions for the solution you need. Good luck!
BTW, your English is fine.