Default [Q] invalid usim after mango update :(

I used Unlocked "LG Quantum ATT ver." in korea.
there has been no ploblem using Quantum except MMS problem.
but 2 days ago from mango update LGQ shows me that I use invalid sim card.
what's the ploblem? why there has been no error when I use LGQ on NODO?
is there any changing on mango or is this just an error?