@pcnr I noticed this issue so many years a go. It may be issues refering undionly.kpxe file. You can find some solutions from this forum about this. In my case I did following:
Make a backup of /tftpboot/undionly.kpxe to undionly.kxpe.bak; Copied /tftpboot/undionly.kkpxe to undionly.kpxe.So at next time, this stuck problem was solved.