Error encountered when connecting to SharePoint Online (SPO) site using Essentials:
1) In Essentials top ribbon, Click Help | Proxy Mode
2) Click Enable Proxy and input 127.0.0.1 for Host and 8888 for Port and Click Save.
3) Close all Essentials tool sessions.
4) Start up Essentials tool again to make connection to SPO site.
Steps for Proxy Mode can also be found here.