PHP Parse error: syntax error, unexpected ‘RouterInterface’ (T_STRING)

This WAS my setup
My Mautic version is: 4.0.0
My PHP version is: 7.4.23
My Database type and version is: 10.3.31-MariaDB-0ubuntu0.20.04.1

But to find why my system stopped sending emails once I upgraded and switched to mjml templates, I restored an earlier setup.

This is MY CURRENT setup
PHP Version 7.4.23
Mautic v3.3.3
10.3.31-MariaDB-0ubuntu0.20.04.1 - Ubuntu 20.04

My problem
Mautic emails are not sending. (They did not send in my earlier setup.)
However, test emails DO send. (As they did in my earlier setup.)

This error is showing in my cron job error notification emails:
PHP Parse error: syntax error, unexpected ‘RouterInterface’ (T_STRING), expecting function (T_FUNCTION) or const (T_CONST) in /var/www/vhosts/example.com/mautic.example.com/app/bundles/MarketplaceBundle/Service/RouteProvider.php on line 15

The only marketplace bundles I have (I don’t remember installing these, must be by default?):
Mautic Focus
Zapier Integration

I did have GrapeJS installed in my earlier instance, but with the rollback it is not currently active.

Steps I have tried to fix the problem
I have been days just trying to find the problem. Now, I don’t know what to do.

So… How do I fix this error? And, what could have caused this error to manifest? (I have restored to a setup my records confirm worked and still I get the error.)

I just reinstalled Mautic.