I have seen intermittent problems  like this. IT happens to us about 1
out of every 100 times. MS is working on a hotfix for our issue.

If it is chronic and repeatable it is probably NOT the same issue as we
have experienced.

Have you ever been able to make the copy? If not, the fix is typically
to cycle services. If that is the problem in your case, you may want to
look at making global groups in NT/W2K and then adding the groups to the
permissions in SPS. Then, when you add and remove members from the
groups, the permissions will take place correctly.

If this is a random thing... Then we need more info.