I assume you are on a console?
Nope PC.
In all my searching and Googling there doesn't seem to be a fix for either as a result of EA's removal of the requirement for the online pass...yet I still have the error when loading up the game that it cannot authorise the Online Pass DLC.
Do you use any mods?
Did you try to repair the game. (in Origin > Game Library > right click ME3 > Repair)
No mods - the DLC works fine on previous save games, just not new ones.
Have not tried a repair, but tbh I'm not sure how that affects it since it's working on other saves and not new ones.
