I found a solution to your problem, as I was having the same problem for about a week. If you are using Mac, then go to Applications -> utilities -> keychain Access. Once the program opened, type iPod. You`ll find the device and login in this list. Click on one of them, the new window will open. Click on show password, enter your Mac administrator password, then you can see your iPod encryption password. Hope it helps you.
If the above can't help you, I also found a solution using an external program to decrypt backup password. You can try to use the application below just download the standard version and check if the password will be detected and will be recovered. It is "iPhone Backup Unlocker".
Get more details from Youtube video to unlock your iPod backup password with the tool:
youtu.be/51uXJI0Rff4