Well, I've successfully removed several other plugins, and AAM is the only one to produce this message. So by deduction, it is relevant to AAM, and it stopping the core form successfully deleting the files and uninstalling the corresponding database structure.
Okay, I will have to perform the manual delete. Thank you for your very time consuming and vert buggy plugin. I'll be sure to recommend it to those who need a partially working user management system.
P.S. I know enough to manage my own simpler implementation, and have had more than enough patience with your plugin (being that I've been making it work correctly since I first installed it over a year ago) YES THAT IS ONE YEAR OF MYSELF SUPPORTING YOUR PLUGIN!
I am going to make a recommendation to WordPress to have this plugin flagged as not stable with the most current version of WordPress. You should not offer this plugin for free with known problems within it. It should be fully tested. I hope you don't offer this same level of quality to those who opt to pay for it. And the WordPress team should take more responsibility in informing users that there is a paid version as well, and should flag the free version as not a fully stable version. It would save us all a lot of time and trouble.