Locked out of encrypted data by mystery password...never set encryption password!

You cannot encrypt something without a password for decryption. You most likely overlooked the part where you were expected to provide a password. Then, before testing that you could decrypt your data, you removed the data.

Unfortunately, there is nothing you can do at this point. Without the password to decrypt the data, the data are unrecoverable.

If you have an employer email account or profile on the device, they can enforce use of an encryption password with iTunes. Usually that is the same as the employer email account or profile, if you did not set a different one yourself with the first backup.

Otherwise, yes, you had to have selected an encrypted backup and set an encryption password yourself. There is no other way for it to get set.

If you see an option to reset encrypted data, several people have posted they did that and lost little to no data.