Share your experience!
I am sorry to hear this,but can I ask you if you backed up on 4.3 and now you want to restore on 4.4?If yes,then problems occur.
I am not sure you will be able to recover them,one way would be to rename the file from .dbk to .rar or .zip.See if that helps.
If you upgrade from one version of android to another, then some files need to be changed. Backing up never takes the backup for installetion file, it takes the backup of all installed one. So changing android can create a problem. Its natural.