Hi @marc_wezdecki, welcome to Community! 
The cause for the issue is that Android does not support chain certificates: Security with network protocols - Android Developers
To resolve the issue, you and/or the server admin can either create a bundle of the SSL and CA certificates to allow Android to shake hands as expected (this would fix it for all users that join in future) or you can refer to this thread for advice on uploading the certificate to the Android device itself.
I hope this helps, but feel free to reach out to our Support Team if you need any further assistance! 