In general this happens when you're building your application using PCL assemblies and from Windows.
I'm not using portable libs,
Ok, still you're likely falling into the same situation. You have the wrong (MS not XI) version of System.Http.Net.dll
being copied (and compiled) on the Mac. E.g.
at System.Lightup....
Is not part of the assembly being shipped with Xamarin.iOS. Having this in your stack trace is an indication that the wrong assembly is being used.
This is a known issue that is being fixed (at least for the PCL part). In the mean time the general workaround is using a redirect file (see James blog) or build from the Mac.
In your specific case you might just need to ensure the right assembly is being referenced.