PGXL Port B Issue

I've owned the PGXL for going on three years and have only now had a need to hook up a second transceiver. The old IC-7851 is on Port A and the new IC-7300 is on Port B. Port A still works as it should. Port B seems to work well in PGXL standby mode -- RX and TX all OK and SWR is good. Then I go to PGXL operate mode -- receiver still receives but the exciter shows infinite SWR and no RF out when attempting to transmit. Both exciters key the PTT line in the PGXL and CI-V is working OK for both exciters.

So I decide to put the old IC-7851 on Port B (with corresponding cabling and CI-V changes) to see what happens. Same problem. Port B gives an infinite SWR reading on the exciter.

I've re-read the manual multiple times looking for an answer. Have I missed something?