I’ve played around a bit and the problem occurs when a user has been new registered but hasn’t clicked the email confirmation link yet. If he tries to log in earlier, this error message appears:
Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 139464704 bytes) in /homepages/**/*******/htdocs/*****/wp-content/plugins/all-in-one-wp-security-and-firewall/classes/wp-security-user-login.php on line 332
Thanks for support!