VS 2015 Community - ALL Controls on all ASPX pages "do not exist" on all C# code behind .cs pages

We moved an active ASP.Net 2.0 website to a new VM server running Windows Server 2012.  We are  using Visual Studio 2015 Community to code but all the controls on all .cs C# code behind pages are underlined with warnings that the control "does not exist".  All textboxes, checkboxes, labels, gridviews.  Every control has a warning not found.   So far all pages work fine online.

Any thoughts what would cause this global error pattern?

Thank you