I have tried with a working MM board from a good known 841 and still the same thing. Maybe it can be a faulty hashboard, but why doesnt the other hashboards work? I think it is uncomon that all the 4 hashboards are faulty,what do you think?
Actually not ECHU shows abnormal status it also shows ECMM[1] which is pointing to MM board issue or bad connection between the hashboard and MM board.
Sometimes if one of the hashboard is faulty it could affect all hashboard so I'd like to suggest to try to run the miner with only one hashboard(Test them one by one) and let see which one is the culprit.
I don't know if Avalon 841 can run with only one hashboard but you can try and update it here.