@robert70569 This is interesting. I think it would fail earlier when trying to NFS mount the images share on the FOG server in that case. Strange.

@sebastian-roth
ok, add another menuentry to /tftpboot/boot/grub/grub.cfg
and add an timeout worked. But there will be a second menu to select one of the two menuentry. Not so nice, but it works.
I could not find a working chain-syntax for the grub.exe.
Embedding a grub.cfg into your core.0 binary I didnt test yet.