Equinemarkt
Failed to authenticate on SMTP server with username "XXX" using 2 possible authenticators. Authenticator LOGIN returned Expected response code 235 but got code "535", with message "535 5.7.0 Invalid login or password
". Authenticator PLAIN returned Expected response code 235 but got code "535", with message "535 5.7.0 Invalid login or password
Your E-mail settings seem to be incorrect (wrong address or password), so you should have a look at that. The error happens because when you post a reply, it triggers an E-mail notification for users that follow the discussion. And if that fails, an error is shown (but the reply is still saved in the database anyways).