I need help with a SQL-query that results in an error
The user you are trying to delete have rows in the feedback table. Given your foreign key definition:

CONSTRAINT fk_feedback_user1 
FOREIGN KEY (user_id) 
    REFERENCES user (user_id) 
        ON DELETE NO ACTION 
        ON UPDATE NO ACTION


You need to first delete from the feedback table:

delete from feedback where user_id = ?;
delete from user where user_id = ?;


An alternative is to change the foreign key to:

CONSTRAINT fk_feedback_user1 
FOREIGN KEY (user_id) 
    REFERENCES user (user_id) 
        ON DELETE CASCADE 
        ON UPDATE NO ACTION


Any rows in the feedback table for that user will then be deleted automatically when you delete a user