Hello Swen,
See if this can help: http://aaron-kelley.net/blog/2012/04/plesk-run-parts-etc-cron-daily-60sa-update-exited-with-return-code-1/
But otherwise according to this http://talk.plesk.com/threads/error-in-etc-cron-weekly-50plesk-weekly.209003/ the respective bug was rectified.