I have a late 2005 A6 with, I think, MMI issues. 

Basically, when I press any of the buttons on the gearstick console, the red text on the dash MMI screen lights up and appears to be attempting to boot up.

After about 10 seconds, there is a quick flash to another text screen (only for a fraction of a second and which disappears far too quickly to read) and then the screen reverts to black again.  A second or so later, there is a barely audible buzz for about 2-3 seconds from somewhere in the vicinity of the passenger footwell (a bit like a servo-motor, perhaps?)

Meanwhile, whilst all of this has been going on, none of the buttons on the console light up and I am unable to use the radio, CD player, or, indeed, alter any of the electronic car settings.

I cannot find a fuse on the handbook fuse panel layout that might have blown and there doesn't appear to be a relay that might be the cause.

Anyone experienced similar problems and can suggest a cause?

 

You say the boot goes too fast to see, here's a little trick, get a mobile phone, set it to camera video mode & film the boot. then play it back slow or step by step frames &see what goes too fast....

 

maybe you can read the boot error.

Not my problem any more!  With the car having appreciated a high mileage anyway and recently starting to experience several other faults that would have demanded serious money to put right, a few weeks ago I traded the car in for a much newer model. 

Incidentally, I doubt the trick that you mentioned would have revealed much of assistance as, one of the things that I did do was to obtain a computer readout of all existing errors.  This was singularly unhelpful!