I used your prog for a while. Nice one! Lately I got the message that the prtscrn hotkeys were occupied by another program.
I tried with http://hkcmdr.anymania.com/ hotkey explorer to find the cause of the problem, but it did not show another tool (see attached screenshot). When I pushed printscreen, it appeared that dropbox suddenly invented a new feature, printscreen to dropbox. See: https://www.dropbox.com/help/1964/en
You can easily disable it by clicking ‘no thank you’. From then it works as designed.
I could not find this info on your website yet.