Original Windows Administrator password.Windows Administrator password (2 users).Old password with different case on first letter.Screen passcode of the first iPhone that was backed up (2 users).Password I use for everything else (2 users).Old password used over 2 years ago (2 users).iPhone unlock passcode at the time the first backup was made (3 users).iPhone screen unlock passcode (9 users).iPhone passcode when corporate profile was installed.Apple ID password, first one ever used, but all lower case (2 users).Apple ID password, but all lower case (8 users).Strongest variation of the base password I have ever used.Computer/laptop login password (4 users).All digit Password created a long time ago.Password used for almost all accounts (3 different users).Old iTunes account password, but not the oldest one.First iTunes account password (40 different users).Please explain how every user that successfully guessed their password had a different one, some of them passwords that Apple's software could not possibly have known? Here are some from from this and other threads on the subject:

I never post on forums like this but I had been trying to fix this for months so thought I would share in the hopes it could help anyone experiencing the same issue. You cannot restore from an old backup but you can simply backup your phone now making sure the encryption box remains unchecked and you now no longer have an encrypted back up. My issue was a little different however, as I still had my old phone so I could simply back it up again once the encryption was removed. After it has reset plug it back it and the encrypted box will no longer be checked. However if you go to Settings, General, Reset, Reset All Settings - it actually removes the encryption (it does not remove any data etc). I had tried every password possible, every suggestion on these forums and nothing worked for me. I definitely never set my back-ups as encrypted and hence never chose a password for a back-up. Not sure if this will help you specifically but it helped me in a similar situation.

Password to unlock iphone backup itunes