We are aware of certain issues with the Defend the Shipment mission which should be addressed in an upcoming patch. We regret we will be unable to provide further updates on this, but recommend checking future patch notes, which can be found here, for any up to date information.
Additionally you can monitor the Public Test Server for potential upcoming fixes and changes, the patch notes for the Public Test Server can normally be found stickied within the Public Test Server Forum. Please note that the Public Test Server Patch Notes are not final, and can be subject to change.
We apologize for any inconvenience caused and thank you for playing Star Wars™: The Old Republic™!
Are you aware that the same/similar issue can also affect "Sabotage"?