Was in contact with Dimitris until the Chat link broke. Last message to him via the Support Dashboard was:
Dimitris, I lost our chat connection.
In the meanwhile, I've tried disabling all plugins, activating a different theme (Twenty Seventeen), and changed the permissions of the WP Contents folder to 777 (was 755). Still no change: the failure on attempted installation of WP Smush Pro still appears.
I also tried installing a Plugin from the WP Repository. It worked OK. So I'm not sure what the issue is right now. BTW: I also returned the permission to 755 for WP Contents folder.