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Hi Stefan, I suppose that the problem is exactly what you say:"heat creeps up out of the heat block and expand the plastic causing a jam".
To avoid this problem you can try increasing the cooling of the hotend, trying to block the heat to come up. Try to use a more powerfull fan.
Do you have an original E3Dv6 or a clone? Is the light or the full version?
Sorry I read just now that you have PTFE inside, so it's the light version...