thank you for your fast reply and the tests!
So there seems to be something wrong at my site 🙁
Maybe it is related to a problem with crontab: At boot the message: “”/tmp/crontab.XXXXYHWFfB”:7: bad day-of-week” appears.
The crontab looks indeed quite messy: http://pastebin.com/VJFuDhBX
Could you maybe share your crontab or a hint what I can delete/change?