Since the original post is three years old, I'm going to go out on a limb and guess that he/she has moved on. Then again, they may work for the government...

Anyway, hopefully this will help somebody: The most likely explanation is that the output spreadsheet is open. If so, the easy fix is to simply close the spreadhseet, and re-run the code. If it's not such an obvious problem, it's probably similar; perhaps the sheet is in a read-only...