I think my problem is solved - also I don't really understand the solution.
Here my situation again in short:
I have two areas (A and B) on my page and both I wanna update with AJAX, independently from each other. In area A there's a tree and I use a poll tag (poll every 5 seconds) and in area B is a drop down menu which is updated by clicking a button.
Although the poll tag specifies which elements should be rendered (only those concerning the tree) the drop down menu is influenced by that too, and we have some funny effects when we try to select something from the menue and the poll disrupt that.
Then I try to use the limitToList attribute in the poll tag. Then I had the effect, that nothing was updated on the page.
Take the limitToList attribute AND place the poll tag near the tags you wanna update with it, so they have the same parent. I don't know why but in my context it works.
Other thoughts, solutions and above all explanations for this behavior are welcome!