A handful of the ~20 pool nodes got stuck mining a super old block... since the nodes were not agreeing, others were mining "empty" (coinbase only), but valid, blocks for quite a while.

Seems there was some breakdown of my alert system and I wasn't notified appropriately.  Some headaches in the rest of my life have kind of made this a lower priority and I, admittedly, hadn't manually checked on things for a while besides a physical look at the servers themselves occasionally.

In any case, I've got the pool server offline for the moment while things are resyncing.  Should be back online sometime in the morning by my estimates.  Will make another post afterward.