I tried to delete a course, that stopped with: Call to undefined method mysqli_native_moodle_database::delete_record()

I found this post: https://moodle.org/mod/forum/discuss.php?d=373044#p1504157

turned on the debugging and got the same stack trace when I tried to delete the course:

Stack trace:

• line 1404 of /lib/gradelib.php: Error thrown
• line 4924 of /lib/moodlelib.php: call to remove_grade_letters()
• line 4840 of /lib/moodlelib.php: call to remove_course_contents()
• line 67 of /course/delete.php: call to delete_course()

I looked into the course and found in the gradebook administration, that the letters are overriden. Turned on 'edit grade letters', turned of the checkbox 'Override site defaults', clicked on 'Save changes' and had the same errors as described in the forum:

Stack trace:

• line 144 of /grade/edit/letter/index.php: Error thrown

But when I leave the checkbox checked, deleted all the entries at the left side, turned the checkbox off afterwards, I could save and the letters where on the default entries,  then I could delete the course.