Invalid or corrupt license

When I try to debug or run I now get the following message.

"Invalid or corrupt license

Please log in with your Xamarin account"

I log in but get an error message. It didn't do this until after I downloaded USB drivers for Galaxy S2.

Also get this: C:\Program Files (x86)\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(2,2): Error XA0000: Could not determine $(TargetFrameworkVersion) for API level '19.' (XA0000) (HelloWorld)