So it seems like you know this already, but * are displayed whenever there isn't a one to one relationship between data when using data blending. For example, if you have item ID 345 in the primary data source, but that item has multiple IDs in the secondary data source, they cannot all be displayed in the same cell, so an * will appear.
The easiest fix for this is to edit the relationships that the data source is blending on to the lowest level of granularity. Editing the relationships can be achieved by going to Data > Edit Relationships... in the top menu toolbar. See http://onlinehelp.tableau.com/current/pro/online/windows/en-us/help.htm#multipleconnections_relationships.html
If this doesn't work, you might try sharing the packaged workbook so that we can see a bit better exactly how the data is structured. Also, try searching the Support section of tableau.com using the exact text for whatever error message you're receiving before the system quits. Many common error messages are well documented and the article associated with it will likely have some good suggestions.
Hope that helps!