This is not a DAVdroid problem. When I try your domain (taken from your email) in the default Android browser, it shows a certificate error, too.
To identify such problems, I recommend SSL Server Test. When you enter your domain there, you get several results (by the way, you should really disable SSLv3 to prevent POODLE attacks).
When looking at the Certification Paths section, you can see that there are two certification paths. In both, “AddTrust External CA Root” is in the default trust store.
- AddTrust External CA Root signs
- USERTrust RSA Certification Authority signs
- Gandi Standard SSL CA 2 signs
- your domain’s certificate.
In one of the certifications paths, the “USERTrust RSA Certification Authority” is already in the trust store, so it’s not required for the server to send it. However, in the other certification path, “USERTrust RSA Certification Authority” is not in the trust store, so it would have to be sent by the server, too.
And this is the reason why your configuration doesn’t work with Android yet: Android doesn’t contain the “USERTrust RSA Certification Authority” in the default trust store (have a look in Settings / Security / Certificates / System), but only “AddTrust External CA Root”. So, you will have to send both “USERTrust RSA Certification Authority” and “Gandi Standard SSL CA 2” as intermediate certificates.