Grub-update Fatal error

Hi everyone, hope you're going fine !
I've made a custom remix of mint 17.3 which is for my students and try to make it iso distributable.
My live CD is working well but when I'm trying to install it the installation fail at the end because it is unable to run grub-update and tells me "fatal error" so impossible to install grub. I'm trying for 3 days to make it work and have red a lot of posts on the internet to find a solution...
Do you have an idea to fix it ? I really would like to redistribute my remix.

Thanks in advance for any clue
best regards

hxc