Hi Boris,
Does a hard-wired connection to the AQ via serial adapter work OK? If so, then there's something in the BT link causing the problems -- maybe BT drivers on the laptop got updated, or something like that? To eliminate QGC as the source of the problem, you could try any other terminal program to connect to the BT emulated serial port and see if that works (you'll see a bunch of junk coming through, but at least it would show that the connection works).
-Max