Sigarr, the Mpenzak branch is the one causing the ritual to fail. You need the branch from the Dark Forest - directly from the Talking tree.
Mpenzak was a trap <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" /> - typical Larian humour. If it comforts you => I fell for this as well in my first game. And I spent countless hrs trying to find out why: this included futile efforts to light/unlight the candles, move stuff around, protect Corinna with 2 globes etc.
BTW, after the failed ritual you see the fake branch lying there - it was not used up due to the ritual. Well, Corinna is a dedicated, not exactly a skilled would-be summoner. This comes from her bad education as an autodidact <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
Kiya
Just in case, you want to do this sub quest => get rid of the Mpenzak branch and trudge to the Tree - try to be either polite there or kill it afterwards.
If you wish revenge:
burn M.'s cart <img src="/ubbthreads/images/graemlins/evilgrin1.gif" alt="" />
and kill the tree only AFTER you have both quests involving it: The branch and another one in the Dark Forest to get an alchemy plant. If you kill the tree too early, you will miss one of these items (they only appear after you get both quests).
