In Microsoft Web Developer 2010 Express I saw this error:
The debug mode is URL but there is no URL specified. Please specify a URL using the project’s Debug property page.
This happened whenever I tried Start debugging after I changed from the 4.0 runtime to the 2.0 runtime, but switching back to 4.0 didn’t help.
Not being familiar with the Microsoft IDE, it took me a few minutes to figure this out. The solution is actually simple. Click Website – Start Options – Use current page.