pfSense

Editing traffic shaper queues (Firewall -> Traffic Shaper -> By Interface) can cause "php: /status_queues.php: XML error: no altqstats object found!" to be logged if "Status -> Queues" is running or is run too soon after committing changes (presumably because the queues are temporarily undefined ... i.e., the error logging corresponds with "pfctl -s queue" temporarily indicating that no queues exist.

If this is "as designed" (due to reloading), then perhaps a note could be appended to the error message, itself -- e.g., XML error: no altqstats object found! (can be caused by traffic shaper changes being reloaded).

This sent me on a "wild goose chase" 'til I figured out what was happening (on the positive side, it was an "expanded learning experience!").