After nearly five hours of sighs, I'm turning to you to get some help.
Here's the situation :
I just bought a refurbished MSI (like new) with windows 10 installed on it.
Since there is not optical drive, I put the ISO of the last ubuntu version (18.04 LTS) on an USB.
I followed this tutorial (in french) : https://www.youtube.com/watch?v=hTIIx9-ZAyw
To sum it up, I shrank my hard drive (I wanted to give 400gb to ubuntu).
I accessed Ubuntu by clicking on update and recovery and selected "boot from a device" option.
I selected "something else" while installing ubuntu. I have created three "parts" : / (with 15gb), /home with 378gb and swap with 6.5 gb. The installation got complete. And I installed it on the windows boot just as the video says to be able to see the grub popping.
Although I must note that when I clicked on "restart" the computer froze and I had to turn it off manually.
When it restarted, the computer went straight to windows 10. I looked it up online and it seems that either fast boot or secure boot was the problem (or both).
I manage to access the bios to disable fast boot but it was harder to find "secure boot" on it. (I have an MSI GL6M 7RDX and the bios seems to pop with "American Megatrends")
And still the grub wouldn't pop up.
So I looked it up online once more and I found this website : https://itsfoss.com/no-grub-windows-linux/
So, I entered this command in windows 10 : bcdedit /deletevalue {bootmgr} path \EFI\ubuntu\grubx64.efi
I restarted the computer and a message saying "invalid signature detected. Check secure boot policy in setup" popped up. But once I clicked ok on it, the grub appeared.
I thought I was out of this mess but no… Once I select Ubuntu on the grub, error messages appear or the computer freezes.
The error messages which are popping are saying :
"ACPI Error : [PEGO.PEGP.EASP] Namespace lookup failure, AE_NOT_FOUND (20170831/psargs-364)
[ACEPI ERROR : Method parse/execution failed _SB.PCIO.LPCB.EC._REG, AE_NOT_FOUND (20170831/psparse-500)
[] ACEPI Exception : Could not find/resolve named package element V0P R (20170831/dspkginit-381)
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Firmware bug : ACPI region does not cover t sponse buffer.
Couldn't get size
/dev/sda5: recovering journal
/dev/sda5: clean, 179442/938400 file, …. blocks
[] failed to load firmware chunk!
[] iwlwifi transaction failed, dumping registers
a couple more lines then the watchdog detect a hard lockup or sometimes a soft lock up
It says stuff like watchdog: BUG: soft lockup – CPU#1 stuck for 22s!
And it just keeps going forever without booting properly…
I've tried to use the "safe mode" and it works. There's a bunch of code appearing and I have the recovery menu. I have tried to repair the files with dpkg, to check it with fsck. Everything appeared OK
I have even use "resume" and ubuntu worked just fine. I have done all the updates it was offering and I thought the issue would be sorted out. I restarted it and tried to access ubuntu the traditionnal way but it started to act up again and to pop error messages.
I can't use the safe mode to access ubuntu forever because it appear very zoomed in and it's… overwhelming to say the least. But I can use it to run any test you would think would help with this problem…
I hope I have been clear enough. I'm really desperate, my last laptop just died a week ago and I would really like to go back using ubuntu…
Best Answer
Looks like I figured it out :
I have found the way to disable the secure boot (it was disguised as "attempt secure boot"). This settled the problem with the safe mode being "zoomed in" but not the other one.
I have found another post where they talk about black screen when first booting. It was because of the graphic card drivers (Nvidia) who were not installed because Ubuntu. Here is the detailed solution to fix these drivers problems : How do I install additional drivers?
Reboot and, problem solved! I post it so, if other people have these problems, they know how to solve them!