I had a problem when I tried to re-activate an order with a backorder entered in a previous period (previous month).  The re-activation worked fine, but I could not complete the Sales Order after making the needed changes. 

The problem ended up being an error in the Accounting Schema.  I had the button checked for Autmatic Period Adjustment.  Once I un-checked this functionality, the problem went away.  I could then Complete the order and generate a shipment to ship the product.