Corrupted backup? —inaccessible, or salvageable data?

I connected iPad 2 first time to iTunes in over 2 years. Wanted to update ios from ios 4 to 6. The hdd was pretty full and I was able to copy pictures and videos to folder on laptop. Deleted cameraroll from ipad and Then I made an encrypted backup. Then I reset and wiped data. Immediately plugged back in, to laptop, then updated to iOS 6. Then connected to laptop again and selected "restore from backup" and entered encryption key.

After three seconds message pops up: iTunes could not restore the iPad "apple's iPad" because the backup was corrupt or not compatible with t...

After trying hundreds of times, including disabling the antivirus for a few minutes still same message every time.

I remember that when i made backup took less than aminute and i was kinda surprised but ther were no error alerts or popups and it said backup complete. i Researched backup folder location and the backup has 525 files and is 258 MB, which is about same as my iPhone backup.

This means backup is not empty, but is it really corrupted or is it some compatibility error, or did I really lose two years of notes, documents, voice recordings and tax records and PDFs?

If anyone is willing to save me from this catastrophic nightmare they have my gratitude and appreciation for your help