A friend just had this problem when upgrading his E71 firmware; he backed up his data using Nokia Content Copier but was unable to restore the data. I thought i should put this up just in case anyone else having same trouble.
After backing up data from the phone using Nokia Content Copier and saved to PC as the .nbu file, the phone was upgraded without any problems and was succesful. Problem occured when doing backup restore using same Nokia Content Copier - the backup file was not found from the application. Even when the .nbu file was found in the PC, the Nokia Content Copier couldn't detect the file. So when the .nbu file was double-clicked, it opened Nokia Content Copier but with an error message; something like "Filetype is unknown. Unable to copy files to phone".
Problem occured with the usage of Nokia PC Suite version 184.108.40.206; Only found solution was to revert back to Nokia PC Suite version 220.127.116.11. Other versions might also work but i haven't tested any.
I have the installer for version 18.104.22.168 with me but its quite a big file to upload, so if any of you need the installer, leave a comment and i'll upload the file.
EDIT: Download the installer from HERE
Hope this helps.