I'm marking this solved from my end.
Here's what I did:
- Used the Zimbra certificate wizard to "Generate the CSR for the commercial certificate authorizer."
- Purchased a certificate from GeoTrust using the CSR.
- Downloaded one of GeoTrust's root certificates in .cer format from their website.
- Once I had my .crt and the root .cer, I went back to the Zimbra certificate wizard and did the following:
- Selected the server name for which I wanted to install the certificate. (Originally I selected "--- All Servers ---" here. I think that is what caused the problem. Once I selected just one of the servers in the list, it worked fine.)
- At the Root / Root CA / Intermediate CA screen I browsed for my certificate (.crt) and the root CA (.cer) AND removed the Intermediate CA, as I didn't need that, clicked next ...
- Waited a little ... and the wizard told me it had been successfully installed.
- Rebooted the server (like the wizard told me to) ... and it now works just fine.
Hope this helps someone else.