We have seen this issue reported before. It usually is connected with an issue in .NET or VS project cache.
Are you opening the website project in Visual Studio?
Generally, we would recommend to try deleting .NET caches:
"$windir\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\",
"$windir\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\",
"$windir\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files\",
"$windir\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\",
(you can delete just the subfolder with IIS site name in the above folders)
After that, please completely close all projects and VS windows and then re-open the project again.
Please let me know if this helps.