As reported in https://moodle.org/mod/forum/discuss.php?d=389064, after upgrading to Moodle 3.7, some forum discussion threads (or a forum activity of type single simple discussion) display an error message:

Coding error detected, it must be fixed by a programmer: Exporter class is missing required related data: (mod_forum\local\exporters\post) author => mod_forum\local\entities\author

Several forum discussion threads on moodle.org display the same error e.g. https://moodle.org/mod/forum/discuss.php?d=65811 If you search for the discussion subject, the forum post is listed in the search results with the content displayed correctly.