Recovering data from an Encrypted iTunes Backup that's missing the Manifest.db
Unfortunately, part of the encryption keys (one for each individual file) is stored in the Manifest.db. The backup password is used to unwrap (unlock) those keys with some more parts from the Manifest.plist, but doesn't help derive the missing key parts. I wish I had better news, but an encrypted backup missing either one of those Manifest files isn't decryptable.

Just to make sure, is there a subfolder inside the backup named "Snapshot"? If so, we should triple-check for the Manifest.db in there.