You might have a corrupted backup in iTunes. So, you'll want to delete any that iTunes may have on record.
To do, connect your iPad to iTunes. Then, in iTunes, go to Edit > Preferences > Devices. Find any/all of the backups specific to your device and, after highlighting them, hit the 'Delete Backup" button. I think iTunes will ask if you are sure ... you are.
When iTunes has finished, try right-clicking and selecting the backup option again. Hopefully, it will work this time. Good luck and let us know how you got on.