Drupal - Fatal error: Class 'Drush\Log\Logger' not found

I had the exact same problem after updating core with drush (installed with composer). This is not something you should do if drupal is installed in the root directory, I've now learned. Reinstalling drush fixed the issue for me.

composer require drush/drush

Next step, I guess if we want to avoid this in the future, is to either move drupal to a subdirectory (as a dependency) or uninstall drush as a composer package and install it globally instead (the old fashioned way).