Change configuration/system.webServer/serverRuntime/asp section of $(SolutionDir).vs\config\applicationhost.config file to:
<asp scriptErrorSentToBrowser="true" enableParentPaths="true" bufferingOn="true" errorsToNTLog="true" appAllowClientDebug="true" appAllowDebugging="true">
<cache diskTemplateCacheDirectory="%TEMP%\iisexpress\ASP Compiled Templates" />
<session allowSessionState="true" />
<limits />
</asp>
Open the website in a browser and then in VS2015 go to Debug->Attach To Process, change code type to Script and select iisexpress.exe. After that in browser navigate to a page you want to debug and Script Documents section will appear in Solution Explorer allowing you to set breakpoints on listed pages. Actually you do all the steps described in this post, except that you edit not the global IIS Express configuration, but local configuration located in VS2015 solution subdirectory .vs\config.
Links:
- Debugging Classic ASP with Modern Visual Studio
- Classic asp “Application debugging is disabled” in visual studio 2015
