This certificate cannot be verified up to a trusted certificate authority.
I can't see any error in what you describe
This is a warning message stating that you have trusted this certificate but CA having generated it is still unknown, which is true BTW until you decide to import in your
trusted CA list the one from Zentyal.
However, such warning message should not prevent you to use trusted certificate.
So this is not clear to me if you are facing any error message or if you are only afraid that it may not work due to this warning message.
If you search this forum, you should find similar topic as I've commented it already a couple of times.
If the assumption and principle supported by Zentyal is that Zentyak works with its own certificate authority, what is for sure missing in the interface is capability for either admins or even end-users to import bot CA and certificates so that applications are aware of it and do not warn users about unknown CA. This is basic certificate management within organizations dealing with internal servers and clients
Of course, request for being able to import external certificates has been expressed multiple times in features request section. That's another story not linked with the problem you face and this is, to me, not mandatory except for services that are exposed outside.
Look at attached screen copy: I'm using Zentyal issued certificates and don't face any error or even warning message