If the chat users list is always empty and the chat is working normally without any scripts errors or conflict the problem could be due to settings or translations.

If you did a custom translations file for the chat ensure that the text used for the search field doesn't contain special characters such as accented characters or non-ascii characters. To test if this is the problem you can set the original text as 'Search'.

Also check if some integration with third party extensions is activated and the setting 'Filter by friendship' is enabled. If so the users list will show only people confirmed as friend.

Ensure that no live support mode is enabled, choosing particular user groups for live support will prevent to show all connected users.

Finally check the parameter 'Max inactivity time to strip out users' and try to choose the value 'Use Joomla session timeout'.