I get error message \"Unable to start debugging on the web server\" in Visual Studio 2010. I clicked the Help button and followed the related suggestions without success.
I don't have a definite answer though these articles may have some helpful info:
Using Windbg to start with w3wp.exe This mentions windbg/cdb debuggers specifically, but this advice should work with Visual Studio talking to cdb (which is attached to w3wp).
Remote debugging with Visual Studio The reason I mention remote debugging at all is that I had to use that a few times in the past to get Visual Studio to attach to w3wp.exe. W3wp.exe runs in a different session, so you can't directly attach a debugger to that.