This is not a problem in Admin Tools. It is a problem with a third party plugin (most likely a plugin either in the user
or the system
group). I can tell because the message you get appears neither in Joomla's core code, nor in our code.
Admin Tools' task to remove obsolete users creates a list of user IDs to delete, then tells Joomla's core code (\Joomla\CMS\User) to delete each one of them. Joomla loads each user and then deletes it.
Joomla calls several events in this process. onTableBeforeBind and onTableAfterBind before and after applying the loaded user data to the user object, onTableObjectCreate when the user object is created, onUserBeforeDelete and onUserAfterDelete before and after the user is deleted.
These events are handled by core and third party plugins. These events do make sense. It's what allows, for example, the User - User Profile plugin that comes with Joomla to delete the additional user profile data instead of leaving them behind. Third party extensions also use these events for similar reasons.
It appears that some third party plugin is experiencing an error while handling one of these events and throws an exception (stop error). Since this happens in third party code called by core Joomla itself we cannot do anything about it. If you try to delete that user by hand you will have the same error. If you do that manual deletion with Debug Site set to Yes in Global Configuration you might receive a more detailed error message which could point you to the extension where this error comes from.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!