In Interdev, click on Project -> Web Project -> Web Permissions. On the Settings tab, is Use Same Permissions as the root Web server checked? If so, then you need to ensure that you have set the c:\Inetpub\
directory in IIS to also allow for debugging (the second option in my most recent post). I remember having a problem with this previously b/c I had set the local app to allow for debugging but, b/c of the permissions, did not have it set that way for the master app/site.
As for the error message, "Cannot attach to localhost", this sounds like it cannot even find the localhost - unfortunately, I've not run into this problem before and am at something of an immediate loss as to why this would occur. Best I can guess is that there is some other (unrelated?) problem with the IIS setup and Interdev trying to connect to it. Are you still able to open up your project like normal? Just as a guess, are you able to setup a new project (which I believe would need you to be able to connect to the localhost)? Not sure of what else it might be at the moment... Worst comes to worst, you may have to uninstall and reinstall, but I cannot help thinking there is an issue with the setup of IIS somewhere... I just had another thought, but now it's flown out of my head just as fast so I'll see if I can track it down and nail it onto a couple of slow-moving neurons... ;-)
You're more than welcome. I'm sorry that we haven't been able to track the problem down just yet. But that is what these forums are for. Goodness knows I've had lots of help from other users, so it's my turn to return the favor(s) - if I can - for others...
For now, hold off on uninstalling and reinstalling just yet. I'm just thinking that the issue lies in IIS somewhere...
-----------------------------------------------------------------------------------------------------
"If you can't explain something to a six-year-old, you really don't understand it yourself."
-- Albert Einstein