After we updated to latest 3.8.x we had the same cron error for assignments module, the fix was to run the CLI admin tool which fixes invalid courseid's, it is
/moodle/admin/cli/fix_course_sequence.php -f
after it runs, it will print out a list of corrected courseid's, your background processor for assignments should run fine after. We have been running on the same database/upgraded over the years since 1.4.x
/moodle/admin/cli/fix_course_sequence.php -f
after it runs, it will print out a list of corrected courseid's, your background processor for assignments should run fine after. We have been running on the same database/upgraded over the years since 1.4.x