Deleting Assignment will not work

Good Morning everyone,

I'm trying to delete an old assignment in a course. This does not work and I get this Error:

https://docs.moodle.org/28/de/error/moodle/multiplerecordsfound

The old Assignment Plugin has been deactivated after all courses were updated to the new plugin a few versions ago. I turned on Debug and this is what I get:

Debug info:

SELECT cm.*, m.name, md.name AS modname FROM {course_modules} cm JOIN {modules} md ON md.id = cm.module JOIN {assign} m ON m.id = cm.instance WHERE m.id = :instance AND md.name = :modulename [array ( 'instance' => '292', 'modulename' => 'assign', )] Error code: multiplerecordsfound

Stack Trace:

* line 1486 of /lib/dml/moodle_database.php: dml_multiple_records_exception thrown
* line 1403 of /lib/datalib.php: call to moodle_database->get_record_sql()
* line 52 of /mod/assign/lib.php: call to get_coursemodule_from_instance()
* line 1686 of /course/lib.php: call to assign_delete_instance()
* line 214 of /course/rest.php: call to course_delete_module()