It's that time of the year to renew the SSL. After couple of failed attempts I started digging around: The error message that Zimbra throws doesn't tell much.
Your certificate was not installed due to the error : system failure: XXXXX ERROR:
Invalid Certificate: /opt/zimbra/ssl/zimbra/commercial/commercial.crt: 
Error code: ZaCertWizard.prototype.installCallback M...
So, just wanted to share my experience with the community since Entrust is not included in the Zimbra Wiki. I am running Zimbra 5.0.13_GA_2791 on Ubuntu 8.04 32bit. (will upgrade soon to 6)

So this is what I've done:

1. I've generated the CSR thru the admin interface (did not tick the box Replace the existing CSR)
2. Sent the CSR to be signed with Entrust
3. Received the signed certificate and the root certificate from Entrust
IMPORTANT!!! You MUST select the jetty web server from the list (If you select any other web server the certificate format looks different and it's not going to work)
Copied and pasted the intermediate, root and the signed certificate in separate files (e.g. commercial.crt, intermediate.crt and root.crt)
4. Once done, click on install certificate, Select the Target Server "ALL SERVERS", then "Install the commercially signed certificate", Next, Next, Select Files and Install.

Hope someone will have use of this