None of the cards in ROC4 were reading out, so we stopped the DAQ to powercycle ROC4. In addition, Vince changed the CRL code, disabling the 1875 in slot 24.
When ROC4 came back up and the DAQ was restarted (with a fresh download), everything seemed to work except slot 20. Since the 1877 in that slot had been recently replaced, it suggested that perhaps the slot itself was bad.
Mark Jones and Charles said they would shift the FPP cards down to not use slot 20 later in the day, which will obvious require the DAQ to be stopped and ROC4 to be turned off.