Sounds like the normal failure of a bearing and then the O-ring so Clutch pack A gets no oil pressure.
I have a Phaeton W12 atm with the same problem and after having fixed one of these gearboxes in my old S8 (now driving around in another S8) I'm finding it hard to get enthusiastic about pulling the Phaeton gearbox out. It's the same design of gearbox.
Does it work properly when the gearbox is stone cold for the first 30 seconds? My S8 did and then failed where as my Phaeton tried to creep forwards in 1 pitifully before going into Limp mode.
Yet another 5HP24 that needs work
MG