@jenesis Assuming you’ve tried every file that might work inside of /tftpboot and have tried several different kernels, then I would direct you to build a boot file with debugging turned on, and then try with that. This is not a fix, but it will provide you with more information that will hopefully lead you in the right direction.
https://wiki.fogproject.org/wiki/index.php/Building_undionly.kpxe