Nice! Thank you @Junkhacker and @george1421
I just changed it to work with the clients MAC address and it seems to work like a charm!
So now, i have this config:
iseq ${net0/mac} XX:XX:XX:XX:XX:XX && goto chainboot ||
goto fog.local
:chainboot
set pxeserver:ipv4 XXX.XX.XXX.XXX
set next-server ${pxeserver}
chain tftp://${pxeserver}/pxelinux.0
exit
I will do some testing but i am verry optimistic 
Thanks!
@Sebastian-Roth thank you for pointing me at the DHCP! I found the problem! All files are fine, but our company DHCP was not! After trying it in a seperated VMware Network it worked like a charm! So all the effort was in vain - i am sorry
We have an old fog running 1.4.4 in our DHCP config for managing the PXE Menu because we are running multiple (i)PXE Services in one subnet. I created a new boot menu item to forward the pxe boot to the new fog server.
like this:
iseq ${net0/mac} a0:36:9f:bd:XX:XX && goto testing ||
:testing
set pxeserver:ipv4 FOG-IP
set next-server ${pxeserver}
chain tftp://FOG-IP/default.ipxe
exit
Got it from here in 2017: https://forums.fogproject.org/topic/9648/add-a-second-pxe-boot-option
Worked fine, until now … And by now, i understand why no certification cmds were availibe because it got stuck in the old fog enviroment. Need sth. to reload the enviroment i think. I will take a look into it to find a way. Maybe i will open a new topic here!
Thank you @Sebastian-Roth for taking the time! Your scrips are fine - it was my fault …
thank for the help! It solved my problem!
Normally the fog installation script set its own password and normally this worked fine to me! Even in the update logs from an earlier SVN version there was a hint to this created user/ password. But in the new installer i cant find this in the logs. After changing it manually everything seems to be fine!
Thanks!