I’m having the same problem as described in this issue on GitHub. The dokku-mongo command to export a dump of the database produces an invalid tar file. Example here:

$ ssh user@example.com
$ dokku mongo:export database > backup.tar
$ tar -xfv backup.tar
tar: This does not look like a tar archive
tar: Skipping to next header
tar: Exiting with failure status due to previous errors