Jens,
My german is rusty, so I'll answer as best I can in english. In order to enable calendar archiving, right click on the "directory" level of the admin gui tree, click on the last tab, and check the IPM types for calendar events.
You may also want to change the advanced settings in your mailbox policies to archive the calendar events the way you want it to.
My advice however, it's usually not worth it to archive calendar events unless you've got a special need.
Anyhow, best of luck.
micah