I am experiencing the exact same error as reported in the forum.
I am also using v2.2.3 of the plugin configured to use SMTP. When I changed to the PHP mail no emails are sent (the hosting company only allows SMTP mail).
Also, only the "Send Newsletter Now" option works; the "Send in background (by Cron)" option does not generate any emails.
I would guess this is a server configuration issue but I could do with some technical pointers/solutions to pass onto the hosting company.
Please note that the issue occurs on a production site and so it is not possible for me to uninstall / reinstall the plugin without a guarantee that the configuration and existing plugin data will not be deleted.