iCal events in mail not added to calendar

I have a MBA and a iMac 27" (late 2013), both running latest El Capitan.

On both i have the same problem.

When i receive an invitation in mail it's not added to iCal.

When i check settings in mail i choose "Add Invitations to Calendar" automatically.

But ...

When i close mail and re-open it is switched back to "never"

It's the same on both machines.

How can i solve this?