I've got the same problem. I have Parallels 8 and am running Windows 7 64-bit on a MacPro with OS X 10.7.5. Was working fine until the last week or so. I'm wondering if the last Parallels update messed something up. I'm going to try reinstalling Parallels Tools and see if that helps.
EDIT:
Uninstalling and Reinstalling Parallels Tools made no difference. Still can't copy from windows and paste into mac. Windows to windows and Mac to Mac copy paste works fine. I submitted a problem report # 18772488.
EDIT 2:
I seem to have fixed the problem by downloading the current version of P8 and reinstalling it.
Last edited: Oct 10, 2012