Light blue screen after F12 on PXE - WDS Server 2012 R2

Hi Experts,

We set WDS on a 2012 R2 server.
Do not listen on port 67 is activated.

DHCP is on another server which is set like this :

43 : 010400000000FF
60 : "PXE Client"
66 : IP of the WDS Server
67 : "boot\x64\pxeboot.com" (Like in the wdsutil /get-server /show:config result for x64)

We tried on a VM, the Hyper-V server is the same than WDS by the way, all is working fine until we choose the boot image (We tried with many boot images (boot.wim from Win7x64 disk, creating  different images with WAIK and so on), the result is always the same...
stuck on a black screen. When we choose safe mode we can see that the disk.sys or ahci.sys driver (depending of the boot image) is not loading.

We tried also on physical computers, the behavior is different, whatever we choose to force F12 or to boot automatically, we need to press F12.
There is no choice of the boot image, a light blue screen appears for a while, then the computer boots from his usual boot order.
 We tried to force approbation by administrator, the F12 key is asked to be pressed and the light blue screen appears...
Nothing in the Waiting for approve tab, so it is not wirking...


Any idea ?

PS : I tried to chanbe the 67 DHCP option with always the same result :
On the the VM, HDD driver is not loading, black screen forever.
On the physical machines, always F12 asked with the light blue screen for a while the normal boot .


Thank you in advance for your help !