Server returned HTTP response code: 503 for URL: <SharePoint URL> (4283515)

While running a Copy Site Job, the following error was encountered: 

!MESSAGE Claims Post Error for URL: - Server returned HTTP response code: 503 for URL:

Error 503 means that the server is unreachable, but returned an error status code which is not related to the Essentials for Office 365 tool.

• Please check server and network connectivity.
• Another alternative would be to use the Admin Account Pooling option - it should help.
• More information regarding Account permissions and security settings in SharePoint Servers can be found here.
• More information regarding Essentials for Office 365 can be found here.