I think this might be the same issue I am getting with latest WordPress (5.5.3) and User Registration plugin (1.9.0):
PHP Fatal error: Uncaught ArgumentCountError: Too few arguments to function UR_Emailer::send_mail_to_user(), 6 passed in /wp-content/plugins/user-registration/includes/class-ur-email-confirmation.php on line 220 and exactly 7 expected in /wp-content/plugins/user-registration/includes/class-ur-emailer.php:271
It happens when you follow the resend verification link when attempting to log in before your email address has been verified.