I tried most of the previous suggestions but in vain. I can't reload the website, so I edited the .csproj file and changed the port number and it worked immediately:
<WebProjectProperties><UseIIS>True</UseIIS><AutoAssignPort>True</AutoAssignPort><DevelopmentServerPort>**4000**</DevelopmentServerPort><DevelopmentServerVPath>/</DevelopmentServerVPath><IISUrl>http://localhost:**4000**/</IISUrl><NTLMAuthentication>False</NTLMAuthentication><UseCustomServer>False</UseCustomServer><CustomServerUrl></CustomServerUrl><SaveServerSettingsInUserFile>False</SaveServerSettingsInUserFile></WebProjectProperties>