Xamarin Studio - can not login - One of the identified items was in an invalid format

Guys from Xamarin Support already post an answer to this http://forums.xamarin.com/discussion/63675/activation-error/p1

In general you should

Update to the "Cycle 6 – Service Release 3" versions or newer. The minimum version of Xamarin Studio that supports MSDN licenses is here: http://download.xamarin.com/studio/Mac/XamarinStudio-5.10.3.51-0.dmg

Log out and log back into your Xamarin account following the "Quick manual refresh steps" on https://kb.xamarin.com/customer/en/portal/articles/1641743-how-do-i-manually-resynchronize-xamarin-licenses-.

Update Xamarin to the latest stable version. That worked for me.