When you say you're installing the certs are you just letting the wizard handle everything or are you installing them to the Trusted Root CA?
Also, it's actually just the Root CA that you need to install. Go into your Zentyal WebAdmin then browse to Mail>Openchange, on there is a big button to download your certs. That's it, just that single crt will do it. Keep in mind it has to be installed to the local user (default for cert installer on Windows) and it has to be installed to the Trusted Rooc CA's (not default, pick choose > Browse > Trusted Root CA) you may be warned about installing an untrusted ssl cert as a trusted Root, yes you want to do that since it's your own cert.
Anyways, glad to hear you got it up and running, keep on reading the Wiki and Zentyal Documentation and you'll keep figuring more out. Never mind the trolls who keep coming in the forums complaining about this free, beta test software not working as they expect. As we are beta testing new, cutting or bleeding edge solutions for Zentyal they are letting us use a fully functional, super solid server product at not cost. Microsoft NEVER does that, I know because I've been a certified partner for 20 years now. I'm also a MOS Master III, a SBS MVP and blah blah blah, they never gave me much more than a few versions of Office for free and it wasn't even the Pro Plus LoL.
Keep up the good work and I hope you get things dialed in soon.