Ok i think i found the problem....nothing to do with Filemaker but a permissions issue on the backup folder. When i moved the backup folder to the dektop i had no problem opening the database.
As an aside: NEVER EVER open a backup file directly. Always copy it to another location before opening it.
The way FMS12 and 13 uses hard links means that if you open a backup directly, you may be changing the same file across any number of backups: