Error Message 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.

Resolution:
To resolve this problem, make sure that the Site URL property value of the SharePoint project in Visual Studio matches the URL assigned to the Default Zone for the alternate access mappings of the web application. Using another zone, such as Intranet, for the URL will not resolve the error. The project’s Site URL and the URL in the Default Zone must be the same. To access alternate access mappings, click Application Management in the SharePoint 2010 Central Administration utility, then click the Configure alternate access mappings link under Web Applications. 

More information Create zones for Web applications.