The trick I used to use to get card stock to print before the firmware update was to take a pencil and slightly curl the edge of the card stock so that the curve end went in to the printer. It also helped to put a stack of them underneath so that the paper was higher up into the tray. I am well aware this is not a fix and I do apologize for that but it is simply a means to get it to work for you. Let me know if this is even an option for you or if this, in fact, helped you.
I really hope this helps you. :)