By my book you're not slow at all!!
Here is the moodlespeak
Debug info: Cannot delete the module taskchain (instance).
Error code: cannotdeletemoduleinstance
* line 1501 of /course/lib.php: moodle_exception thrown * line 198 of /course/rest.php: call to course_delete_module()
I have bee too quick at syaing they were the same (oops).