Hi Lawrence,
Is it just affecting one page/page type or is it for any pages you try to copy? If its a particular page that wont copy, are you using any custom JavaScript on this page? I'm wondering if there could be a conflict between custom scripts and Kentico scripts.
Outside of that, does it happen when you test this locally vs on the server? Maybe there is some environment factor that is involved. Lastly, it may help to check the Event log for any other errors in case there is some other issue that could be involved.