To workaround the issue, please open the Terminal.app on the Mac workstation and run the following command:
sudo chmod 0644 /Developer/MonoTouch/usr/lib/mono/2.1/monotouch.dll.mdb
This issue should be resolved in an upcoming release of Xamarin.iOS. However, if the issue remains on a newer version of the software please e-mail firstname.lastname@example.org with your full versioning information and full build log output. Alternatively, you may post to the bug report tracking the error to let our iOS Team know that the issue remains.