Late response I know, but I had the same problem. Installing the CA for both RapidSSL and GeoTrust on the server-side solved it for me.
http://support.servertastic.com/rapidssl-and-geotrust-certificate-not-trusted-on-mobile-device/
This is the RapidSSL and Geotrust CA bundle you need.
https://knowledge.rapidssl.com/library/VERISIGN/ALL_OTHER/RapidSSL%20Intermediate/RapidSSL_CA_bundle.pem
Documentation of some providers:
- RapidSSL
- Goddady