Random users are unable to login using the Moodle Mobile App. Instead, they are presented with the following error message:
"Invalid response value detected"

Steps to Replicate:
1. Connect to the moodle mobile app
2. Enter test username and password
3. Try a different logging in as a different user until the below error message is received: "invalid response value detected."

Replicated on 3.1.1

Additional Investigation Notes:
The below forum mentions User Quota being an issue. Checked client's user quota to see if non-numerical values were in the Site Policies > User Quota area, and there were not:
https://moodle.org/mod/forum/discuss.php?d=320873

Moodle mobile FAQs also mentions user quota: https://docs.moodle.org/30/en/Moodle_Mobile_FAQ#I_get_the_error_.22Invalid_response_value_detected.22_when_accessing_the_site

22 Aug, 13:14 Test Login - System System Web service function called The web service function 'moodle_webservice_get_siteinfo' has been called. ws 99.38.164.214
22 Aug, 13:14 Test Login - System System Web service function called The web service function 'core_webservice_get_site_info' has been called. ws 99.38.164.214
22 Aug, 13:14 Test Login - System System Web service token sent The user with id '199489' has been sent the web service token with id '21'. web 99.38.164.214

Client Impact: 3 - High impact for users affected since they are unable to use the Moodle Mobile App.