What's happening really doesn't make any sense.
I just encountered this issue today. I purchased an iphone 6 and backed up my iphone 5 to itunes and I have changed my old encryption password to a new password. I thought that I made a typo during the change password process and somehow I typed it twice to something that wasn't in my head.
Their is a way to view that password on mac from applications/utilities/keychain access and then view the password.
it says that I have chanced the password exactly an hour ago and when I ticked the show password button it showed me the password that I was trying to type all day long. I copied and pasted that exactly same password and it kept saying incorrect password.
I still have both my phones and I can do the backup on another computer but still it doesn't make any sense!! there is a bug in itunes backup encryption!!
Could the problem be different IOS on the old iphone and the new one ..... i'll try to update my old iphone and see if that will solve the problem.
Cheers.
Hussain