XPS 9350 boot issues after Support Assist update

Hello everyone,

Yesterday evening I downloaded the latest version of Dell Support Assist.

Installed it, launched it then I let it run the tests and updates. All done.

I reset the PC and at startup it opens up Support Assist again, to run some more tests. 

All tests passed, but an error message appear, saying that there is no bootable drive detected.

Indeed the XPS can't boot.

Switched it off. Turned it on pressing F12 to open diagnostics. Tests ran and everything is fine (beside the battery, which is at 59% of its original capacity after 5 years).

The drive is detected and showed as working fine but the XPS can't boot from it.

Switched it off, then on pressing F2 to open settings. 

Checked everything, tweaked some and ran more on-off procedures to see if it could boot. 

It couldn't. Restored previous settings.

Following the suggestion of a thread: added a new boot option, bootx64.efi.

Nothing.

Knowing that the drive is perfectly fine and that the system just wouldn't boot I did this: opened the bottom panel of the laptop, unscrewed the drive, mounted it in an external case and tried if it could boot from it then, when connected via USB cable. And it worked! It was slower but it did boot up correctly. 

Now: what do I do to make it boot normally when the SSD is inside the laptop and not connected from outside?

I'm here for more details if needed.

Thank you.