Well it might be due to a non reinforced hdmi link. I switch to dvi and it's working fine.
When i tried without displaylink hub i started with no problem for few weeks and black screen happens again (weird i dont understand why it did not happen immediately).
With an adaptator hdmi=>dvi connected to displaylink usb (now 2 weeks) i do not have problem any more.
sorry for the waste of time :/
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