I found the issue: yesterday I had been trying to see if another monitor was working on this particular machine, and shut it down.
The thing is: this particular monitor had precedence over all others and so, gdm was displaying the "primary" screen to this monitor. Since it was shut down, I was not obvious that it was taken into account. Anyway, sorry for the inconvenience. It seems that all the "NVRM: Xid" errors were linked to this monitor being connected and not turned on, because there is no errors of this kind now. Sorry again for pulling the trigger too soon. Cheers, JB
