You might want to reverify your SMTP settings: host, port, and authentication. It should work. Make sure your encryption settings for the SMTP match what the server wants to see
This is a pretty specific issue, my advise would be to pay someone to fix it for you. https://www.phplist.org/paid-support/ It probably won’t cost very much, the priority should be to get sending again ASAP.
You might want to update your version of php, the newer version seems to require an upgrade.
Also, you cannot just re-use the 3.0.6 config.php. Some of the settings changed, you would want to edit the new version of config.php that comes with 3.0.12 to the settings that you had in the 3.06 version.
It’s got to be something simple, it should work fine.