A few months back I complained that my HP 4M had become a tar baby, just sitting there, refusing to print any file sent to it. "Nothing was wrong", but it wouldn't print. In desperation, I pulled the firmware memory card and reseated it.
Bingo. It worked.
This just happened again. Again, "nothing was wrong" (that is, the diagnostics showed no problems, and yanking the cable produced an instantaneous "connection loss" message). And again, when I pulled and reseated the firmware memory card -- that fixed it.
Curioser and curioser... Perhaps the contacts need a bit of ProGold?