正如问题中已经提到的，重新安装所有 Xamarin 也不起作用.卸载 Visual Studio 2015 有效.以下步骤成功了(至少对我而言):

As already mentioned in the question, re-installing everything Xamarin also did not work. Uninstalling visual studio 2015 worked. The following steps did the trick (for me, at least):

1) 卸载 Visual Studio 2015 测试版.

1) Uninstall visual studio 2015 beta.

2) 卸载 Visual Studio 的 Xamarin.

2) Uninstall Xamarin for visual studio.

3) 重新启动系统.

4) 重新安装 Xamarin Studio.现在，可以使用 Xamarin Studio 和 VS2013 进行编译.

4) Re-install Xamarin studio. Now, the compiles with both Xamarin studio as well as VS2013.

PS:这是最后的解决方案之一.任何更好、更准确的答案总是受欢迎的.

PS: This is one of those last-resort solutions. Any better and more accurate answers are always welcome.