@kleber I’d recommend removing the background image via the GUI under Settings | iPXE General Config. I just removed the background image name from that field entirely. The default background prevented certain machines from being able to boot for me as UEFI. Figure its a quick enough thing to check.
Are there other machines that do boot ok with your setup or are you not able to boot any machine via iPXE?
Do the machines you have issues on have any BIOS settings specific to PXE booting (enabling it, authentication, etc)? If you you may want to check the configuration there.
Have you tried UEFI boot instead of BIOS boot, I have found sometimes the error is more descriptive in one vs the other (or even that one way has no error and the other does). I have found it especially helpful to have a camera/phone camera with high speed video recording, ie 120/240 fps and record the screen after I make my selection to catch any brief messages on screen.