I have a problem. I got the letter from the Dark Brotherhood, told me to meet some girl. Did that, got the quest to put some snake venom into some wine. Did that too. Then I went to the woman I should report the success of my mission, but she didn't talk to me. When I click at her in Grab- or Dialogue-Mode just nothing happens. When I look at her, I'm told her name, which is identical with the one in my log.

I've never experienced that. Have you checked your Affiliations to see if DB comes up now that you've clicked on her?

No, I'm not in.

Edit: Ok, thing's just taken care of itself. I had to put the wine back on the ground, after I put the venom in it. I had no idea, it was in my inventory. Now I also understand what the "Now all you have to do is put it back" message meant.

Correct, she won't talk to you unless you completed the quest, which does require you to drop the poisoned wine

Correct, she won't talk to you unless you completed the quest, which does require you to drop the poisoned wine

How true...I remember after I whacked my first wiseguy, my padrone just stared blankly at me until I'd put the empty clip on the table.

Well, since you were to poison the guy, walking off with said poisoned wine just won't do.

Yeah, I know. But I didn't know you have to put the poisened wine on the ground manually. When the wine on the ground disappeared, I thought "Well, that was easy." and went out, and didn't notice it was in my inventory.