If a user receives a notification from a forum post, they navigate to the notification page (within the app), view the notification. Click on reply in the notification, type a message, submit the message and then receive the error:

Could not add your post due to an unknown error.

I get the same error on iOS and Android (Moodle Mobile version 3.2.0). Moodle application is on 3.1.3 (Build: 20161114).

I can navigate to the course in the app, go into the forum and successfully reply to the forum post.

Attached images show:

mm01: display of notification on Moodle Mobile App
mm02: reply message in Moodle Mobile App
mm03: error message