I've found the problem.
I read that GRUB can be placed inside the LVM and that it is somehow possible, however with the lack of proper documentation I DO NOT recommend trying this. That is what caused the problem, placing GRUB inside LVM volume on RAIeD-5.
To fix this, all you need to do is write GRUB to the regular HD's, outside of the RAID partitions. You can do this be "chroot"ing, but I have found an easier way:
Boot from Live CD, and make sure you have internet connection, then:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair -y
This will launch a GUI repair tool, follow the instructions carefully and it will re-install GRUB for you.
The lesson is do not place GRUB in an LVM volume upon installation, choose one of the hard drives, outside the RAID partitions i.e. /sda, /sdb/, etc...