Here's the error I'm seeing:
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Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: The "civi_flexmailer_required_tokens" service or alias has been removed or inlined when the container was compiled. You should either make it public, or stop using the container directly and use dependency injection instead. in Symfony\Component\DependencyInjection\Container->make() (line 275 of /home/sites/dev/vendor/symfony/dependency-injection/Container.php).
Any pointers most welcome.