When you "delete" a user in Moodle, it sets the value in the "deleted" column in the user table to 1. All the various processes in Moodle then know to skip this user as deleted. Not all the user data is actually deleted, but some may be retained. In any case, it will not throw any errors for standard Moodle.
I do not know of a plugin by that name, but that's where the trouble lies. If it actually removed the user from the table but did not clean up all the references to the user in other places, then it did a bad job, since clearly there are still references to it in the cohort members lists.
I do not know of a plugin by that name, but that's where the trouble lies. If it actually removed the user from the table but did not clean up all the references to the user in other places, then it did a bad job, since clearly there are still references to it in the cohort members lists.