It is safe to delete this file using shell. Anyway you should not keep any backups in moodle data directory for security reasons. Automatic backups should be stored outside moodle web and data directory. Teacher backups should be erased after backup or restore.
We have fixed many file and backup related issues in 1.4.1+, similar problems should be prevented in the future
skodak
Backup and restore
Backup filename errors!
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