It seems that we have found a bug in the forum activity that only affects the App. (This problem doesn't occur in a mobile browser.)
 
When a student using the movile app accesses a Q&A forum thread that already has replies, the app returns a "Invalid response value detected" error message. Turning on debugging yields a more detailed error message:

discussions => Invalid response value detected: posts => Invalid response value detected: userposts => Invalid response detected: timecreated => Invalid response value detected: Invalid external api response: the value is "" of PHP type "NULL", the server was expecting "int" type

FULL STEPS:

1. In an existing course as TEACHER, create a forum of the "Q and A forum" forum type & add a new question to the forum.
2. Access course as STUDENT 1 & answer the question in the Q and A forum.
3. Access course as STUDENT 2 using the mobile app & attempt to answer question in Q and A forum.

WHAT IS EXPECTED:

Student 2 answers forum question and reads other student replies.

WHAT ACTUALLY HAPPENS:

Student 2 receives an "Invalid response value detected" error in App and cannot answer the question in the app. (The Q and A forum functions as expected if there are no student responses. That is, Student 1 did not receive an error message even if answering using the app.)

WORKAROUND:
Opening the forum in a mobile browser will allow students to post to the forum. Tap the 3 dots in the top right corner of the screen and then tap "Open in browser".
 
We have reproduced this problem on our server also on our MoodleCloud account.