I am no expert in such situations, and that is why I have spent the last 5 hours trying to find out how to restore my backup with a password I either didn't create or I forgot... still, I tried all the possible passwords I am using and have used in the past but with no success. The only backup available from iTunes WITHOUT a password was one from 2016, so all of the new data, pictures, messages of the past 2 years where gone.
I went through all of your posts, YouTube channels, etc, trying to figure out what to do and finally decided (by some strange faith) to check Keychain Access, and there I found the password I have apparently used for a backup made back in 2016 - I don't even remember creating one, but I did and it worked finally!
Now, I am not sure why Apple is not suggesting among all of the options it offers to also check Keychain where you might find your forgotten passwords? Instead you are warned that "There is no way to recover your information or turn off Encrypt Backup if you lose or forget the password." !!?
Perhaps Apple has this information somewhere, hope I just missed it? But if it really is not there, then Apple, PLEASE do include all the possible options/suggestions available, this was a very time consuming and irritating experience, and by what I read below here, I can see I was not the only one finding it hard to find my way around... Thank you.