Thanks Littlemews,

The light-echo scenario is explained in detail as you quoted, but if you remember the time-lapse "movie" it is immediately clear that there are no previous shells being lighted (look at the sequence and try to see different shells). What is visible is one shell expanding, there is structure in the shell (strings and holes) that grow in size, exactly what you expect when one shell is expanding. Also the event was very fast, the star brightened 10.000 times in only one day! This is not expected in current theory, so the event is thought to be very rare. But since we see it, it can't be that rare, maybe this is exactly what every nova looks like in reality, not in theory.
Cheers.