While doing two snapshot metadata compare an Error "Unable to find a metadata backup for folder name" is prompted

This is because one of the SFDC 2.0 snapshots was backed up without enabling

“Enable Metadata Backups” setting under “Backup definition”

You can detect which snapshot doesn't have the metadata by doing the following

Under Metadata tab observed under View, Compare and Restore Snapshots

Enter one of the snapshot date and time and if that snapshot doesnt contain the metadata following error is observed

Error ! No updates found for the selected metadata.Select a different snapshot

Now if you compare the snapshot which doesn't have the metadata along with another snapshot that consists of metadata then the following error is observed

Make sure under the backup definition the following setting “Enable Metadata Backup” is always kept in the enabled state and save the backup definition

Once this is done from the next backup snapshot you will see the metadata is backed up and you will be able to compare the metadata of the latest snapshot with any other snapshot that has got metadata backed up within it.