When you create a custom page that is located on www.example.com and your board is inside www.example.com/community/, viewonline will display users on the custom page as if they were on the index page of the board.
The problem is this preg_match function in viewonline.php:
If the saved session_page looks like ../test.php, this preg_match will just remove the session_page and the $on_page will be set as empty array.
For the time being I just changed the if statement directly afterwards from this:
But that is just a mean fix to me. It's probably best to change the preg_match regex.