Hello,
you should mention your Moodle version: it could be a bug of an old version or a new one e.g. due to some special characters used in the "name" of one/some of your users.
You should debug it by inspecting the actual HTTP Response sent by Moodle, using the Developer Tools of your browser to help understanding why such a missing (the ,) on listing your users.
HTH,
Matteo