I tried but there isn't any more updates available. It's running latest firmware already.
Did you do the firmware update by hardwire or Wi-Fi?
I suspect that somehow your firmware update got corrupted. It is not a good idea to do firmware up dates by Wi-Fi. I never do.
The problem is that you can not remove the update, I don't think. I suspect that will require return and reflashing of the BIOS, but I can not be certain of that, but strongly suspect that is what needs to happen.
Alternatively you can just do without Audyssey and do a manual set up. I find that Audyssey is a quality destroyer anyway. I have used on a number of systems in a couple of homes now, and it downgraded all of them substantially. I have three systems here and the Eq part of Audyssey decreases SQ substantially. It is useful for speaker leveling, distance and as a guide to crossover point.
Yo can work around it with an spl meter, and a program like REW or omnimic. I think both of the latter are far preferable to Audyssey.
I really doubt your issue is a hardware problem and pretty much for certain is a software problem. I did a search to find other reports of this and could not.
It is just possible that the latest firmware released has this bug, and there will be more reports. I would call Marantz and see if they have had other reports of this problem.