There is no reason why you cannot do an iCloud backup. iCloud backups, by definition, are encrypted, and do not require a passcode.
Once you have backed up your device to iCloud, then hook up to iTunes, and erase your phone there and back it up as a new device.
Then restore it from that backup in iTunes one time to ensure that you no longer need the password.
Then try restoring from your iCloud backup, and then back that up to iTunes to see if it will back up as an un-encrypted backup.
Also, use every iTunes or other password you can think of to try to crack the encryption password. There is no limit. Use really old iTunes passwords, really old lock screen passwords, combinations like 1234, 9999, 0000, your cat's name, your dog's name, your kid's name, your first teddy bear's name....anything you can think of. Undoubtedly, this happened awhile ago. It does not ask for the password when you back up (after you have set the original one), so unless you are trying to restore from it, it won't ask, and if it has been years since you set the encryption, it is very easy to forget - especially since people see a request for a password, and often just type it in.
You can just back up to iCloud going forward if you cannot get around the encrypted password.
Best of luck,
GB