Quest is totally buggy and makes playing the game unpossible. But I don't know which mod starts it so I can't deactivate the esp. When I try to deactivate all in hope to find the right one, the game crashes. Please help.

try deactivating all the esp's and have only the oblivion.esm running whist ur out of game after u deactivate load up oblivion...
if this doesn't work i reckon it's a texture file or something u've recently put into ur data folder just delete mods u've recently put in. If u cant remember just open the mod u put in and cross reference and delete.

If none of this works reinstall i guess....

Do you mean the 'Caught in the Hunt" quest? If you do that's not a mod. But it is known to be 'buggy'
You can find out about it, its bugs and how to avoid them here.
http://www.uesp.net/wiki/Oblivion:Caught_in_the_Hunt

If its not this quest, ..er.. sorry.

@predator: I have hundred of mods installed, that's the problem. *g*
@Glue: Sadly this is not the quest I mean. It is called Hunt for the Hunters. Has something to do with Werelizzards. Pretty strange.

Thanks anyway =)