After this answer, I can connect to the IIS Express instance with debugging.
However, I can place breakpoints in a small set of my ASP pages, as shown in VS2012 Solution Explorer, while the debugger is connected to the IIS Express process of the website:

These are basically the first ~ 20 ASP files in the site directory hierarchy.
If I try to set a breakpoint in any other ASP file in the solution, I get a tooltip:
Currently, the breakpoint will not be deleted. No characters have been uploaded for this document.
How to enable interrupt debugging feature in all ASP files on a website?
debugging visual-studio-2012 asp-classic iis-express
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