Getting a strange error when visiting some pages on a D9 Civi 5.38 site related to Symfony container and flexmailer tokens

Question

Here's the error I'm seeing:

The website encountered an unexpected error. Please try again later.

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.

Answer 1

I wasn't aware that Flexmailer had moved from being a contributed extension to being a core extension, so consequently went ahead and installed the contrib extension as I normally would when getting Mosaico installed. So I ended up with two copies of the extension, which led to the error. To fix it I disabled Mosaico and Flexmailer extensions (need to disable Mosaico as it depends on Flexmailer), uninstalled the Flexmailer extension, deleted the extension files from my contributed extensions directory, then refreshed my extensions listing, installed and enabled Flexmailer (which is now listed as a core extension - i.e. the path to the code shows that it is a core extension), and then re-enabled Mosaico.