|Error The Teacher Assignment file contains an invalid course. The teacher is XXX, the course is 3573.|
Error Invalid related course found in the Course Link file. The course is 1003, the related course (invalid) is 1004.
|Error Invalid course found in the Course Link file. The course is 1083.|
For all of these courses, the first thing I would check is that all the course settings are correct, in particular the one that says "schedule this course"