As far as it was explained to me there is some ‘address’ that PCI devices get, and that address has a certain amount of digits or whatever. So quite simply, the earlier versions of UAD-2 don’t conform to a current standard, and there was no way for UA to do anything to make them work. Physically there wasn’t enough space to fit assigned addresses. Something like that. So that applied to the solo, duo and quad cards.
The Octo however was a later device and never had this particular problem as far as I know.
So I’m not sure what happened to fix this but I think perhaps motherboard makers updated their BIOS to accept shorter address names or something along those lines.
If you want an actual explanation that’s detailed and certainly more correct than mine then you can search forums like the UAD-2 forum, Gearslutz and maybe ASUS, and search for uad-2, “address” maybe… something like that. Sorry for being vabue.