I had exactly the same problem. No password set during backup etc...
exchange password didn't work, 1234, 0000, 9999 didn't work, current Itune password didn't work as well.
:)I solved by google search "How to Disable Backup Password in iTunes", and get suggestions.:)
To disable my iTunes backup password when I met prompt:
"The attached device has a backup password set. You need to disable the backup password in iTunes before you can continue. Start iTunes, remove the backup password and start this Program again".
I got tool "iPhone Backup Unlocker" to help me with its "Brute-force" Attack mode to remove backup password in iTunes.