Can't run crontab without excecuting the programm once
Finally solved my problem. Everything was fine, but i was using the root crontab. The root crontab was not able to find the display, before the display was not mentioned/used by another command. I transferred my cronjobs to the "normal" crontab and everything works fine. Another point is that commands, which need the display (for example Tkinter) do not work if you start them @reboot. You have to implement some sleep time (~30 seconds) in your script, so the display has time to become available.
import timetime.sleep(30)...