Just a heads up to others that may get hit by this.
I’m a one-man show and run Mautic for my own businesses marketing.
I recently updated to v4 and with that came a requirement to update to PHP 7.4. I had changed that on the hosting side but what caught my up was I had CRON jobs configured running on PHPv7.3. Trying to run Mautic on a PHP7.3 server will throw a fatal error and nothing works.
The big problem was I saw no errors anywhere telling me that the CRON jobs were not working and that campaigns and segment updates and all the other things PHP-driven CRON jobs were failing. Even manually running the CRON job via SSH didn’t return an error.
Once I updated the CRON to run on PHPv7.4 everything started to work again.