Hi!
I think you have to share your workflow, or at least some more description about the different elements.
Can you reproduce, at which element the error occurs?
Can you create a backup-copy of the workflow, and add some "tracer ammunition" System.debug or System.log entries?
Can you find something in server.log or scripts-log.log?
(file location: see http://www.vcoportal.de/troubleshooting/
)
Regards,
Joerg