This error is thrown. Posting here to report.
PHP Fatal error:  Uncaught Error: Call to a member function get_error_message() on string in /home/site/wp-content/plugins/newsletter/profile/profile.php:98
The error stems from this instruction on line 98$this->redirect($this->message_url($user, $email, $res->get_error_message()));
I’ve patched it by removing the 
->get_error_message()
 part. if 
$res
 is not an WP_Error object, then it wont contain the 
get_error_message()
 method.
Might also be good to add a 
break;
 after line 95, within the if statement. Not sure how the 
redirect()
 method works but if 
$res
 is an WP_Error object, there is no need to go further in the switch statement.