From "MDL-26960:"

Obtained the following message:

Invalid get_string() identifier: 'messageprovider:message' or component 'mod_feedback'

line 5891 of /lib/moodlelib.php: call to debugging()
line 6481 of /lib/moodlelib.php: call to core_string_manager->get_string()
line 222 of /message/edit.php: call to get_string()
Also 'Course creation request approval notification','Course creation request rejection notification' and 'Feedback notification' settings were listed, even though course requests and the feedback module are not enabled for the site.

Finally there was a setting listed as '[[messageprovider:message]]'.