At which point in your process does it fail? Do you have any tasks running for the change at the same time?
Have you looked at your log file? What kind of logging do you have enabled?
Have you checked any Add Reminders in the process. It maybe a case that if you have an auto add reminder being used in a process referening to a placeholder or a value type for the source to be empty and therfore have a null in the subject line.