Re: Master and Commander Achievement & Extended Cut Error
At what point did you download EC?
Beginning "Priority: Cerberus Headquarters" locks down your EMS, so jumping in after that mission might cause an issue (I haven't tested this, so it's a bit of a guess).
"There seems to be other issues with the mix of Leviathan DLC and Extended Cut DLC that do not allow the extended cut scenes to play after completing the Leviathan DLC."
The only thing I've experienced are the additional flash forwards (when you ask for more information about your options) not playing, which end up giving Shepard an odd little head tick. I read somewhere on BSN that this doesn't happen when you start a new game with EC and Leviathan installed, but I haven't tried it yet.