This seems to happen whenever I'm vendoring items, I have repeated it several times and this has only occurred after this last patch.
When vendoring items back (low level stuff no one would buy) the screen darkens offering up a selection box as in "Are you sure you want to sell XXXX for XXXX? Yes / No" screen. You select the answer and it lightens up and you can go about your game.
What happens now is that the screen darkens, asks you the question, you answer - it will then blink lighter and darken again or in some cases just stays darkened. No box will show. Your mouse will move about the screen - as in move around - but you cannot select anything, move or function in the game. Only hitting the ESC button to get the main menu screen up works and you need to select Exit to reboot the game to cure the bug.
Choosing anything other just shows that screen darkened up with no ability to select anything - requiring you to once again hit the ESC key and access the main menu screen - which shows up undarkened.
Minor annoyance, happens only when vendoring items. Not during the game.