dual boot - Installation fails on Grub (Read only EFI partition?)

I have a brand new Dell Inspiron 5000 with Windows 10. I've shrunk the windows partition and tried to install an Ubuntu 16.04 LTS to have a dual boot.
The installation (sda8 for / and sda9 for /home) runs well until it tries to install grub. At this point it fails with a "critical error" and I'm not able to boot Ubuntu.

Trying to repair grub with boot-repair I have the same error.
I've tried installing Ubuntu 14.04 LTS with the same result.
Investigating a little I've seen that if I try to mount sda1, this partition is always read only. Is this normal? Any idea on how can I complete my dual boot?
This is the paste bin from boot-repair:

http://paste2.org/gCPyJFvg

By the way, I've turned off the fast boot in Windows 10.

First make sure that hibernation is disabled - so boot into Windows, open command prompt as administrator and execute : powercfg /h off. Shutdown the PC completely - do NOT reboot.

Then install GRUB boot loader to your Ubuntu installation in EFI mode.
Boot from the Ubuntu installation media, open a terminal and execute:

sudo mount /dev/sda8 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done
sudo chroot /mnt
grub-install /dev/sda

update-grub  

Note : sda = disk | sda1 = efi partition | sda8 = system partition | (taken from 'paste2' output)

Boot into BIOS - change the boot order in UEFI settings - select Ubuntu to be the default system.