That error seems unrelated.
In general, it’s easier to test “email in” than it is to test reply-by-mail. Check the manual polling enabled and email in settings, add an email address to the staff category and see if you can send email to that using the same email address you’re using for your forum admin user account.
Then check Admin - Email - Bounced / Received / Rejected to see what is going on.
Is your reply by email address configured correctly?