To test this issue you need to be able to modify the database of the site and change the code. In web, login as admin, go to Site administration > Development > Debugging and set "Debug messages" to DEVELOPER. Create a course and enrol at least 2 users in it. Now access the site database, go to the user table (usually mdl_user ) and add some HTML to the firstname of one of the users from the previous step (e.g. <b>Terry</b>). In the app, login as the other user (the one that wasn't modified) and go to the course from step 2. Click "Participants". Check that you see an error message saying that an invalid response was detected, but no details are provided. Go to the More tab and then to Settings > General. Enable "Display debug messages". Go back to the Participants and perform a PTR. Check that now the modal displays more details about the error. Now modify the code of  get_signup_settings in auth/email/classes/external.php and add this line: throw new Exception('Test'); In the app, go to add site and enter the URL of the site you modified the code. Click to create a new account. Check that you can see the stacktrace in the error modal.