Ran into this as well. It might even be expected:
Not all web browsers have implemented the Root CA for SHA2 certificates. Consequently, it may be necessary to add the cross-signed certificate below to your server so that the client can verify the certification chain. …
I had followed the instructions to add a cross-signed certificate. Unfortunately, the bundling order in Gandi’s instructions are backwards. I didn’t have any problems with Firefox interpreting the cert chain but SSL Server Test revealed it was “incomplete”. Reversing the order fixed the problem and immediately DAVdroid connected without complaint.
The cert bundle, in correct order, looks like this:-----BEGIN CERTIFICATE----- MIIFdzCCBF+gAwIBAgIQE+oocFv07O0MNmMJgGFDNjANBgkqhkiG9w0BAQwFADBv ... 0fKtirOMxyHNwu8= -----END CERTIFICATE----- -----BEGIN CERTIFICATE----- MIIF6TCCA9GgAwIBAgIQBeTcO5Q4qzuFl8umoZhQ4zANBgkqhkiG9w0BAQwFADCB ... BT02Vf6Dsuimrdfp5gJ0iHRc2jTbkNJtUQoj1iM= -----END CERTIFICATE-----