System.Diagnostics.Debugger.Debug() stopped working

Extracted from here (MSDN) the following note:

Starting with .NET Framework 4, the runtime no longer exercises tight control of launching the debugger for the Break method, but instead reports an error to the Windows Error Reporting (WER) subsystem. WER provides many settings to customize the problem reporting experience, so a lot of factors will influence the way WER responds to an error such as operating system version, process, session, user, machine and domain. If you're having unexpected results when calling the Break method, check the WER settings on your machine. For more information on how to customize WER, see WER Settings. If you want to ensure the debugger is launched regardless of the WER settings, be sure to call the Launch method instead.

I think it explains the behavior detected.

I finally figured out what was happening. For some reason, something changed on my machine so that just calling Debugger.Break wasn't sufficient anymore (still don't understand what changed). In any case, I can now cause the debugger to come up by using:

if (Debugger.IsAttached == false) Debugger.Launch();