Class Joomla\Plugin\Behaviour\Taggable\Extension\Taggable not found
From Joomla! Documentation
Updating from a version below 4.2.0 sometimes leads to an error that says Class "Joomla\Plugin\Behaviour\Taggable\Extension\Taggable" not found. The result is the page is broken, see screenshot.
4.2.0 Update from previous version - Taggable not found.
This pertains only Joomla pages that updated to Joomla! version(s): 4.2.0
What is the cause
The cause of the issue is still being investigated. The autoload_psr4.php file is not removed on the update. The current state can be found in the GitHub Issue
How to fix
Remove the following file: /administrator/cache/autoload_psr4.php
The system will then generate it again and the error is gone.