Error when debugging: "Could not load web.config file. The given key was not present in the dictionary."

Could not load the Web.config configuration file. Check the file for any malformed XML elements, and try again. The following error occurred: The given key was not present in the dictionary.”

If your configuration file is well-formed, possibly this error is happening due to your public URL set on Central Administration, on Alternate Access Mappings. The Site Url property of your project in Visual Studio needs to match the public URL on AAM. Check it on Central Administration > Configure alternate access mappings > Edit public URLs.