Why doesn't my Nexus 7 synchronize since Android 6?
Meanwhile, i can answer my own question. Found a description from Tasker that explains and solves the problem here http://tasker.dinglisch.net/userguide/en/androidpowermanagement.html

The important part for my Nexus 7 with Android 6:


  The location of the setting may vary per device, but a common method is to launch the Android Settings app, select Apps, click the cog icon top-right, then Battery optimization. Click All apps, then Tasker, then Don't optimize.


Didn't know this menu, good to know for other apps too.