I can confirm that this bug is affecting me too. MAG256 with latest firmware. It works great, up until you enter the YouTube app. As soon as it starts, a white "mouse cursor/icon" ends up in the middle of the screen.
There seems to be no way to control the cursor (try to move it into bottom corner but no go), and I cannot turn it off either.
The MAG256 remote control is different than the 254 and 250. I see no mouse button on my remote either. Pressing any button doesn't help.
Think its a bug that needs to get fixed on the Infomir side of things.
Cheers