Hi Nick, so i tried a bit, but what i found out is, that it is not a cable issue. It is a software issue of the H8000.

When i go to my RME control panel and switch from WC "Master" to "Autosync" and then back to WC "Master" the H8000 becomes the right clock and the "ext" light is no more flashing.

Here is a link to a picture of my RME controlpanel.

https://docs.google.com/document/d/1_dH1MxVkup7QMmmoOHCExJv0WMS9jl0yk1gTSmIv8Rc/edit

If you still dont believe me i can make a little movie and show it you and the world on youtube.