'ASP.NET Configuration' Feature Not Working

Hi

I recently formatted my Windows 8.1 laptop and installed Windows 10. Since reinstalling Visual Studio 2012, I am no longer able to access the 'ASP.NET configuration' feature via the project menu. This is a problem for me, as I use it to control my web app's users, roles, etc. When I select the .net config option, a web browser opens, but I get the following error...

Server Error in '/asp.netwebadminfiles' Application.

Compilation Error 
  Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately. 

 Compiler Error Message: CS0122: 'System.Configuration.StringUtil' is inaccessible due to its protection level

Source Error:


 

Line 987:
Line 988:             // Put together some unique app id
Line 989:             string appId = StringUtil.GetNonRandomizedHashCode(String.Concat(appPath, appPhysPath)).ToString("x", CultureInfo.InvariantCulture);
Line 990:
Line 991:

Source File: c:\Windows\Microsoft.NET\Framework64\v4.0.30319\ASP.NETWebAdminFiles\App_Code\WebAdminPage.cs    Line: 989 



Compiler Warning Messages:


Show Detailed Compiler Output:


Show Complete Compilation Source:



Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.6.106.0 

Any idea what's going on here?

Thanks

Steve