[MAC] Connection to Communication Service was lost...

This issue can be solved by clean reinstall, please follow instruction below to do that:

Please remove the GOG GALAXY Client app from the Applications directory.

Please also remove the contents of these directories:
/Users/Shared/GOG.COM/Galaxy
$HOME/Library/Application Support/GOG.COM/Galaxy/Configuration

You can open it using the "Go to Folder" command accessible with the Command+Shift+G key combination or selecting Go > Go to Folder from the Finder menu.

Now restart your Mac, download the current installer from:
https://www.gog.com/galaxy
and reinstall the GOG GALAXY Client.

Don't forget to import your games back after following above, instruction how to that can be found here.