Hello again Tim,
I tried myself to recreate the solution following the steps a couple of times, and I got the same problem as you’re experienced.
What worked for me was to remove the copied Tobii.EyeX.Client.dll from the root while Unity was running and then simply run the game to force it to copy the DLL again. After that, everything seem to work as expected. Not sure why this happen, but I guess it has to do with Unity caching the old version of the DLL somehow. To be honest, I have no clue but I will investigate this further.
I’ve also updated the conversion steps (moved #14 to #1).
Best regards
Patrik