I just completed a Drupal 10 upgrade and I'm getting this same error when I try to add a new element to an existing policy. It worked without problems during the entire development process, but now that I've pushed the upgrade to production it fails.
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. Valid plugin IDs for Drupal\symfony_mailer\Processor\EmailAdjusterManager are: mailer_url_to_absolute, email_bcc, email_body, email_cc, mailer_default_headers, email_from, mailer_hooks, mailer_inline_css, email_plain, email_priority, email_reply_to, email_skip_sending, email_subject, email_theme, email_to, email_transport, mailer_wrap_and_convert in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of /var/www/web/core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php).
I should add that this is a policy for email receipts in Drupal Commerce.
Drupal 9.5.8
PHP 8.1.19
Rules 8.x-3.x-dev
I updated to the 8.x-3.x-dev version as suggested, which fixed the deprecated function errors but now none of my rules work as before. I tried deleting and recreating them, still not working. Anyone else have this experience? Any suggestions for how to get Rules working again?