VS2015 Website Compile-debug issue

I recently moved a web site from VS2013 to VS2015.

If I compile with no aspx or aspx.cs or web.config documents open, I compile fine (no errors). As soon as I open one of these documents, errors pop-up, which stop the compile (and debugging).

I can start debug with no docs open and then open them after debug starts. The solutions works properly.

Are there any solution or project options that can stop this from happening.

Thanks

Abbott