If you do a ctrl printscrn button press in windows, and then try pasting into paint - do you paste an image in there (ie, is the printscreen button working?).
If so, then maybe some settings have gotten messed up - I'm not big on keymapping, but inside your :
Microsoft games\Freelaner\Data\Interface folder you'll find a file called:
keymap.ini
ini files are encrypted,if you haven't got the sdk then trying to open it may look like a right mess - in which case you'll need the sdk.
Once you install the sdk over your freelancers folder (it should hopefully locate the data folder itself i think), then you can open the file in a text editor.
Look for this section:
[KeyCmd
nickname = USER_SCREEN_SHOT
ids_name = 1995
ids_info = 3318
state = keyup
key = 44
Check that it's under this secton:
[KeyMap
nickname = IDR_ALWAYS_PRESENT
key = USER_SCREEN_SHOT
If it is, and it's the same - then i dunno. try remapping to another key - perhaps
[KeyCmd
nickname = USER_SCREEN_SHOT
ids_name = 1995
ids_info = 3318
state = keyup
key = "L"
Then press L and see if it takes a screenshot. I'm sure something somewhere allows for the location of screenshots to be chosen, but I don't remember where.
You could also just try and reinstall instead