As I wrote I have wiped out the Trisquel installation on this PC. I will do a
test installation on my Main PC of Trisquel 6.0 to an USB stick, update and
see if Nouveau fails here also. My main PC has the same pre-requisites since
it is a dual GPU PC.
Even if switching of the onboard GPU solved the issue it is hard to
understand for my why an onboard GPU should interfere with the main PCIe GPU.
To my best knowledge most onboard GPUs get automatically diabled when you
plug in a dedicated GPU in the slot. You will find not the slightest evidence
that there is a dedicated GPU available. Neither via dmesg, Xorg.0.log or
hwinfo.
This might be different if the chipset and GPU supports parallel use for
onboard GPU and dedicated GPU. I know that ATI has some chipsets which
support this.
