Fatal error: Uncaught Error: Class 'Civi\FlexMailer\Services' not found

Overview

After enabling the flexmailer extension, I get an exception whenever I try to load any CiviCRM page.

See my comments below about my particular environment - this may turn into more of a support request than a bug...

Reproduction steps

1. Enable flexmailer extension.

Current behaviour

I get the following exception:

Fatal error: Uncaught Error: Class 'Civi\FlexMailer\Services' not found in /usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/ext/flexmailer/flexmailer.php:147
Stack Trace
	flexmailer_civicrm_container()/usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/CRM/Utils/Hook.php:271
	CRM_Utils_Hook->runHooks()/usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/CRM/Utils/Hook/WordPress.php:136
	CRM_Utils_Hook_WordPress->invokeViaUF()/usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/Civi/Core/CiviEventDispatcher.php:168
	Civi\Core\CiviEventDispatcher::delegateToUF()/usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/vendor/symfony/event-dispatcher/EventDispatcher.php:214
	Symfony\Component\EventDispatcher\EventDispatcher->doDispatch()/usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/vendor/symfony/event-dispatcher/EventDispatcher.php:44
  	Symfony\Compon in /usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/ext/flexmailer/flexmailer.php on line 147/usr/local/www/wordpress/wp-content/plugins/civicrm/civicrm/Civi/Core/CiviEventDispatcher.php:129

Expected behaviour

No exception should occur.

Environment information

• CiviCRM: 5.32.2
• PHP: 7.4.3_
• CMS: WordPress 5.6
• Database: MariaDB 10.4

I am running in a WordPress Multisite environment - the civicrm installation is the same for 3 different sites in the multisite install, and that includes flexmailer. This issue only happens on one site in the multisite setup, which leads me to believe that there's something wrong with that site's configuration, but I can't figure out what that might be.

Comments

There was a bug previously opened that seems to match this:

https://github.com/civicrm/org.civicrm.flexmailer/issues/61

However it doesn't appear to apply to this version.