I'd say you have got a clone chip and havnt ended up with the latest drivers (perhaps windows re-installed the current ones allready?) The description of what happens when the fake chips are detected from the link 2 posts above is:

"It seems that in this case Chinese designers implemented protocol-compatible "fake" chip, using mask-programmable microcontroller. This way they only needed to redo 1 mask - this is much cheaper than full mask set, and explains a lot of redundant pads on the die. Fake chip was working kinda fine until FTDI released drivers update, which were able to detect fake chips via USB and send only 0's in this case."

I think that because you are seeing all nulls in realterm, that shows that you are hitting this issue still.