so far removing an account is working, but when the user clicks the "Unregister me" link, he is first asked if he really wants to and then this message appears:
"There is some error in sending request to admin. Please contact them by using any other means."
The account is removed after that, but the user has seen an error message and will not be sure if his request worked. This text can't be edited in the language file, so it is probably a system message. I disabled JQuery and Bootstrap in Joomla 3.5.1.
checked email settings in site configuration. Testmail sending works, all other components, plugins and modules using PHPMail function also work. Disabled "Email as system email" in plugin configuration. But no effect.
The account is removed (of course also in database, checked it), so the plugin works so far on the side. But the user, who is sending the request will be confused seeing the error message.