Hi Jurgen,
Could you offer us the detailed information about the workflow for further troubleshooting?
Meanwhile, you can troubleshoot this issue referring to the similar thread below:
http://social.technet.microsoft.com/Forums/en-US/sharepointcustomizationprevious/thread/64e3bb7e-305c-433e-8753-f1cc3ee4e2fa
Thanks,
Edward Qu