I have run into this - it's a character limit in the slushbucket field.  Navigate to Workflow -> Activity Definitions in the left nav.  View the "Catalog Task" record.  On the "Activity Variables" related list, personalize the list layout to add the Max length column.  Note that the "task_variables" record has a maximum length of 4000.  Make this bigger to accommodate more variables in the slush bucket. I have used a value of 10000.