Receiving the following on latest version of Matomo (3.3.0)
/opt/piwik# ./console queuedtracking:print-queued-requests --queue-id=1
Showing requests of queue 1. Use --queue-id=1 to print only information for this queue.
WARNING [2018-02-21 19:50:11] /opt/piwik/plugins/QueuedTracking/Queue.php(86): Warning - Invalid argument supplied for foreach() - Matomo 3.3.0 - Please report this message in the Matomo forums: https://forum.matomo.org (please do a search first as it might have been reported already)
Just set up redis clustering, but not sure if related.