Hi all
I work with a project based on “Blank app Xamarin.Forms (Shared)” and am on the way to try ship my app to the shops (what is a pain b.t.w. 
).
By my tries, I abruptly had a new (never seen before) error-message to my iOS-project, was not able to build anymore and have lost almost a day now to fix the problem…
I then have searched the internet for every peace (word) of the error-message and found a Stackoverflow message to Visual Studio (without Xamarin-Tag)
The solution was, to uncheck the checkbox “Activate ClickOnce security settings” on the security-tab to my iOS-project.
I never have changed anything on the security tab explicitly and also don’t see the hang-together with the error-message
Therefore I post the error-messages here, hope this message is found and help someone… 
Example error-message in German:
"app.config;obj\iPhone\Debug\MatrixGuide.iOS.csproj.MatrixGuideiOSapp.exe.config" ist ein ungültiger Wert für den ConfigFile-Parameter der GenerateApplicationManifest-Aufgabe. Es können nicht mehrere Elemente an einen Parameter vom Typ "Microsoft.Build.Framework.ITaskItem" übergeben werden. C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets
Example error-message in English:
C:\Program Files (x86)\MSBuild\12.0\bin\Microsoft.Common.CurrentVersion.targets(3243,9): error MSB4094: "app.config;obj\x86\Debug\EnergyMS.csproj.EnergyMS.exe.config" is an invalid value for the "ConfigFile" parameter of the "GenerateApplicationManifest" task. Multiple items cannot be passed into a parameter of type "Microsoft.Build.Framework.ITaskItem".