After upgrading to Visual Studio 2013, I receive the following warning when I edit the markup of an ASPX or ASCX file, and the designer file does not update with the change. I have ReSharper 8.1 installed. I have tried cleaning the solution, restarting Visual Studio, clearing the Resharper cache, and restarting my computer without any luck.
A colleague of mine ran into the same issue, he was able to resolve this but simply suspending resharper and enabling it again.