There is a known issue with Essentials where you will get a "Sequence contains no elements" error when loading a site that has unreachable map services for the first time. If Essentials is ever able to reach the offending service, then this error will go away.
To avoid the error message, you can navigate to a different page when loading the site. When you load the site in Manager it will try to launch a page on the Map panel:
If you replace "Map" with "SiteInfo" we will not try to initialize the map object model, and won't encounter the sequence with no elements:
From here you should be able to fix the service connection issue and load the site successfully once the map service issue is resolved.