this part of the Deneb USB FAQ is interesting, worth a try with those having an A3640 and RR NAK issues (disabling CPU cache):
DENEB works only without CPU caches
Some rare hardware configurations show a mixture of problems while operating DENEB: Poseidon can't start the card driver, USB devices are not recognized (or only after restarting Poseidon in Trident), USB operation is unstable.
Most configurations with these problems have a Cyberstorm Mk1 or Mk2 involved, sometimes also A3640 are affected. Problems change or vanish after resorting Zorro cards in the Zorro backplane or by adding/removing the CPU card SCSI module.
Root of this problem is that the CPU cache is activated too early by some card ROM, in most cases CPU ROM. This can trigger a bug in the expansion.library. You can workaroung this problem by activating the "nocache" module inside the DENEB FlashROM. More information on this module can be found inside the Poseidon and Luciferin guides.