Is this a self-signed cert where you've changed the Locale/City/ST/OrganizationalName?
-Then it might be
Bug 23515 - generating a self-signed certificate with non us locale fails which as been fixed for 5.0.2 (zmcertmgr deploycrt self -new)
If you're talking about a commercial cert you might look at:
How to manually install your commercial certificate in 5.x - Zimbra :: Wiki