Might just be coincidence, but I ahve read that Excel can try to use the root of the MyDocuments drive to create temp file when copying sheets. If you don't have access rights to the root (as is often the case with a network location), it fails with an error like the one you described. Of course it may also simply be a sign of corruption. If it's just one sheet having the issue, try copying all the cells to a newly inserted one and see if that fixes it. If so, it's probably worth thinking about creating a new copy of the workbook!