Ah! The dreaded error code -50! You use to see this one alot with P3 but not so often with P4. It's a catch all error code so has several different causes. If memory serves (like I said it's been a while with this one), one possible solution that covered several problems was to delete the Poser.ini and let P4 rebuild it. You'll probably want to save a copy of the .ini under a different name just in case.