I was told by 4 people at Apple Support that I must have entered a password and not remembered it.
And the thread here proves this to be accurate. In each case, a user came back to tell us they found their password on their own.
They can't take their noses out of their canned response manual to listen to anything I had to say. Was told there was nothing they could do with the encrypted backup file.
Not a canned response when it is the truth. Nothing you can say to Apple Support, or here, will alter reality.
We can be empathetic to the frustration you feel when locked out of a backup. As this thread proves though... Apple did not place a password on your encrypted iTunes backup.
In the short term, for troubleshooting like this, you can use an iCloud backup, which is already encrypted and will store heath and homekit data, plus passwords with no extra password needed.
I strongly believe that if their program can create a backup file and automatically create a password for it, some program engineer knows what password was created or knows a way to find out.
Again... iTunes does not and did not automatically create a password for you.