I don’t know what were your particular issues, but I used to have problems with it not being able to home it’s motors and even failing to unload and load a cleanly cut filament. All it said is “MMU REQUIRES USER ATTENTION”. It was very frustrating for me, prints not being able to start etc. And of course the idiotic error message.

But about three weeks ago they released a new firmware version for both the MK3S and the MMU2. And I was surprised to see that the printer started displaying normal error messages and even a short link to the exact documentation page which explains the error condition. And basically mine was this: http://prusa.io/04306, which basically boils down to “Power issue detected”. I took a look into the mainboard box and indeed it was a loose connection on the positive terminal. I’m glad the thing didn’t melt or catch fire (the printer was a kit, which I bought second-hand, assembled by some random guy).

Anyway, you might want to put back your MMU, update it, maybe some of the issues will be better explained to you.