Thanks for your answers. Just had no time to response until now.
I did a backup and tried the crossupdate. After the update I couldn't receive any more TV channels. I found out that TVHeadend needed an update. After doing this update the TV channels startet to run, but after some seconds it is frozen. That's why I had to go back to my backup with Openelec.
There are some configurations I changed right now in my Openelec TVHeadend setup. I will try to do the update again in the next days (when I have time to) to see, if the problem will continue or not.