I found that was wrong, it was one host that was blocked on start on fog without any task pending on it ??
IP 172.19.81.167.

We have to find what host ( there is 800 computers in school) without having his hostname.

I have to compare export of hosts in Fog with mac adress and the apache log file.

After i have to find in my GLPI inventory where was that netbios name in the school rooms.

The computer failed to start on PXE. I shutdown it too quickly to save the error message.

After shutdown it, i delete again with mysql command blank images and fog runs fine again.

Thanks for your help searching in apache log ! i understand how it works now.