By deleting the ".meta" folder you remove the meta information about the backup
With the "Backup metadata" option checked you will later be able to restore mailboxes from the backup. Otherwise – you will not
Rename the metadata file to its standard name or recreate the metadata files.
Backup to unmanaged vault fails if the XML file with archive metadata is corrupt
The following conditions are met:
File comments and metadata (i.e. color labels) are stored in separate ways on Windows, Macintosh OS 9 (and earlier) and Macintosh OS X. For this reason, comments can be lost in a cross platform environment.
Some metadata is stored unencrypted in an encrypted archive
The operation fails because the archive meta information is damaged or missing. Run vault validation to fix the issue