@aitala @dcoppel I had the same issue as the one you outlined here https://www.drupal.org/project/rules/issues/3469170 🐛 Using token annotation in rules action crash after update to version 4 Closed: duplicate .
In the send email portion of the rule I had left the language field empty as it wasn't required but after upgrading to version 4 it started throwing errors. I would suggest checking if you have any empty fields that could be returning null instead of a string.
For your reference, I have set the language field to @language.current_language_context:language_interface.