Hello Joost,
yes, it's possible that the certificate is causing problems, specially in Android.
First of all, certificates using TLS1.2 aren't supported in Android 4.3 or older.
Also, the certificate must be correctly configured in order to work. You can use a SSL Checker to verify everything's ok.
You should try to open the site in your device's browser (Chrome for Android, Safari for iOS) and check if it's opened right in there.
If you still have problems please send us your Moodle URL and we can take a look.
Kind regards,
Dani