I may sound like a noob, but this problem is really not getting out of my mind, you should try it too.
whenever i add Microsoft.Phone.dll to a new project of Windows Phone 8.1 for the WebBrowserTask control, i get the following error.
And i have really no idea how to solve this.
In previous versions (Windows Phone 8) it is fully functioning, but as we all know there are many changes in the WP8.1 SDK, therefore, i am not able to fix this issue.
If you use Windows store apps project (WP8.1/Win8.1), WebBrowserTask class was replaced by Launcher class.
Now you must use LaunchUriAsync method from Launcher class
Some useful resources :