My 1dx back screen gets black when flash is plugged

[hafoto]

Hey,

I recently got a 1dx (mark 1), when I plugged a flash the back screen gets black and no button responds. I'm just able to change aperture and shutter speed (even not iso), and make picture.

Once I turn the flash off, the camera gets fully functional (sometime I have to restart it), and I can see the picture I just made.

I tried with a old 580ex and a 580exii, same experience. With my 5dm2 I never had such issue.

Do you guys have any idea about what's wrong ?

Thanks.

[hafoto · Accepted solution]

I figured out, the M-fn button was stuck inside.
After pulling it out I've been able to use the flash.