I was looking for a way to  export the timelapse for a second time but with different settings without doing the time-consuming export from LR again. I already have my folder (LRT_...) with the tifs. How can i make the LRtimelapse rendertask-window pop up?

Just go to File/Render Video in LRTimelapse, there you can select the intermediary sequence (LRT_) and set different render settings.