The problem I described earlier on 16.0.0RC2 is also happening with 16.0.0RC3. If I open the batch manager in single mode with Pending community uploads in the list I get the following error.

Warning: Undefined array key 16 in /var/www/html/piwigo/admin/batch_manager_unit.php on line 409

https://piwigo.org/forum/viewtopic.php?id=34529