You likely have dirty data in your FOG DB. The easiest way to find the dirty data is to enable remote access on the DB and then connect to the DB using a graphical DB viewer. I would recommend HeidiSQL for this: http://www.heidisql.com/
There are some pointers for enabling remote access on the DB here: https://wiki.fogproject.org/wiki/index.php/Troubleshoot_MySQL Forgive the roughness of the article, it’s under construction currently.
Once connected to the DB, just take your time and look closely at all the data inside the hosts and hostMAC tables. If anything glaringly stands out, make note of that entry, and then just delete that row.
Each row of host data that you delete will mean you need to go and re-register that particular host (eventually).