When I delete a course, which is no longer used, I get this message:

Error reading from database

More information about this error

Debug info: MySQL server has gone away
SELECT count(r.repositoryid)
FROM gquj_files f
LEFT JOIN gquj_files_reference r
ON r.id = f.referencefileid
JOIN gquj_backup_ids_temp bi
ON f.id = bi.itemid
WHERE bi.backupid = ?
AND bi.itemname = 'filefinal'
[array (
0 => 'f36156b9b3f6a3d6901f69ed13246292',
)]
Error code: dmlreadexception

Stack trace:

• line 494 of /lib/dml/moodle_database.php: dml_read_exception thrown
• line 293 of /lib/dml/moodle_read_slave_trait.php: call to moodle_database->query_end()
• line 1278 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->query_end()
• line 1679 of /lib/dml/moodle_database.php: call to mysqli_native_moodle_database->get_records_sql()
• line 1752 of /lib/dml/moodle_database.php: call to moodle_database->get_record_sql()
• line 1962 of /lib/dml/moodle_database.php: call to moodle_database->get_field_sql()
• line 522 of /backup/util/dbops/backup_controller_dbops.class.php: call to moodle_database->count_records_sql()
• line 2221 of /backup/moodle2/backup_stepslib.php: call to backup_controller_dbops::backup_includes_file_references()
• line 34 of /backup/util/plan/backup_execution_step.class.php: call to backup_store_backup_file->define_execution()
• line 181 of /backup/util/plan/base_task.class.php: call to backup_execution_step->execute()
• line 191 of /backup/util/plan/base_plan.class.php: call to base_task->execute()
• line 145 of /backup/util/plan/backup_plan.class.php: call to base_plan->execute()
• line 567 of /backup/controller/backup_controller.class.php: call to backup_plan->execute()
• line 132 of /admin/tool/recyclebin/classes/category_bin.php: call to backup_controller->execute_plan()
• line 185 of /admin/tool/recyclebin/lib.php: call to tool_recyclebin\category_bin->store_item()
• line 5056 of /lib/moodlelib.php: call to tool_recyclebin_pre_course_delete()
• line 68 of /course/delete.php: call to delete_course()

The result is I can't delete a course.