Through chat had @jrm run a wget to download the bzImage’s from the fogproject server. OP stated these kernels worked.

Bonjour
Oui c’est bien FOG 1.2.0 et c’est bien la modification initrd=$imagefile qui à fait fonctionné sur MAC

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Hy
Yes it’s good FOG 1.2.0 and it is changing initrd=$imagefile who in fact worked on MAC

Sounds a bit like you have two DHCP servers answering clients’ discovery requests in your network. Sometimes they pick up IP and PXE information from FOG and sometimes they pick up whatever the other DHCP server provides (probably only IP without PXE info)??? Just a wild guess…