Do you have access to the database behind the website, to check and make sure the "introformat" field is in your mdl_lesson table? Because it should be.
Another thing you could try is to Purge the Caches. Sometimes when database changes are made, you have to purge the cache for Moodle to bring them through. I don't know what change could have been made to mess that up, but it's worth a try at least.