As you know, theres been a couple of posts/threads now on here whereby of users on 20CN have had problems with PPPoE.
I know that the old 20CN platform used to emulate PPPoE by actually using PPPoEoA. MSiP is gradually being shut down - in fact its mostly gone now.
On the latter link you posted, Mike was on 20CN and then suddenly last week found his exchange had only just been upgraded to 21CN.
Aside from the shut down of MSiP, BT are also making some other major changes to their network, and thats replacing the old bRASes and introducing
MSE bRAS, although the 20CN exchanges are expected to be the last to go, but it depends on location. If your exchange is small then it may now be piggybacking from a nearby larger exchange.
Im not certain, but it would seem that BT do seem to be piggy backing more & more exchanges. My own exchange has been 21CN for years with its own backhaul.. yet I found out that just this year they changed the routing so that now all my traffic goes to via a neighbouring town... so there are some major changes going on right now.
Why this should cause problems with PPPoE though Ive no idea.