Ok so I got over this strange bug, while creating a pbuffer it appears that wglChoosePixelFormatARB from Forceware drivers does not like some DCs, even if the DC passed there should be useless. It simply fails with GetLastError 0xc0070591. 0×591 appears to be ERROR_DC_NOT_FOUND.

So beware what DC you send, I finally sent one that has a OpenGL RC associated and it worked fine.