I saw in the tracker that this error has come up several times and had to be fixed by the core moodle team. In my moodle 2.0, when some one tries to request a new course, the request goes through, but it gives an error message:
Coding error detected, it must be fixed by a programmer: Could not load preference message_provider_course_courserequestrejected_loggedin. Does the component and name you supplied to message_send() match a row in message_providers? Message providers must appear in the database so users can configure how they will be notified when they receive messages.
I get this type of error when I try to approve or reject the course, as well. The course does show up in pending requests and if I approve it, it shows up under miscellaneous. But, then I have to go to the course and move it to the right place since the second screen never shows up due to the error. Rejection of a course never works.