I tested various QR codes from version 1 to last version 40 on many websites (mine as well - works fine) and it brings sometimes ERROR, but I'm sure it is not because of this add-on. It is because of the way each QR code is placed into a page or the code itself brings error because of incorrect encoding!

Just install and open a page with QR code, right mouse click and from the popup menu select then "Decode QR"... and test it yourself.