I believe I have just run into this bug https://bugzilla.xamarin.com/show_bug.cgi?id=4902 on my Nexus 4 now that I have updated to android 4.3.
Basically, when JSON.NET tries to get the timezone, we get a null reference exception because there are issues with an environment variable in certain android builds.
The same code worked fine before I updated and still works fine on other android devices. Are there any workarounds available for this? Is there anything else I can do or am I stuck? Has anyone else run into it recently?