I faced the same issue and from code I found that the link is conditional and it's checking the user profile option "Screen Reader".
The solution is to just edit the user profile and set "Screen Reader" attribute to "No".
if (empty($USER->screenreader)) {
$params['id'] = $chat->id;
$chattarget = new moodle_url("/mod/chat/gui_$CFG->chat_method/index.php", $params);
echo '<p>';
echo $OUTPUT->action_link($chattarget, $strenterchat, new popup_action('click', $chattarget, "chat$course->id$chat->id$groupparam", array('height' => 500, 'width' => 700)));
echo '</p>';
}