Hi Dennis,
I believe I have resolved this issue on your site previously. The issue is related with your translated url from "entry" to "Einträge" where this string does not get translated in email url. You can refer to my reply here,
http://stackideas.com/forums/cron-1#reply-227223 . Unfortunately this fix are only applied with the new post created right after the fix was made (2nd October 2015). You can try to create a new post and then check your email notification to see if the url is generated correctly or not.
Hope these help.