Web Requests fail on Android 4.3

I've got a mobile app that hits up a web service for various things. Everything had been working fine on iOS and Android 2.3 -> 4.0 (that's the range that had previously been tested) but a few days ago the app was deployed to a 4.3 device. On 4.3 the requests fail with a "Error writing request: The authentication or decryption had failed" message. I've done a bit of searching and found across these posts:

http://stackoverflow.com/questions/12287528/webclient-ssl-exception-with-android-4-and-mono-for-android https://bugzilla.xamarin.com/show_bug.cgi?id=6501 http://forums.xamarin.com/discussion/2653/authentication-or-decryption-has-failed-for-webservice-over-https

but so far nothing has helped.