Thanks Jacob and Bzz. I think I found the bug.
Just as in this topic Dynamo Player executing frozen nodes. Our work-in-progress script did contain older frozen versions of the plot node. (always a good idea …) After adding a watchnode after the frozen node and with output in de player, I could see that the wired and frozen version was actually working in de ‘headless’ Dynamo Player enviroment. I could reproduce this behavior in Revit2020. But not in Revit2021 and Revit2022. So I do (hope) this is fixed in newer versions.
In our office multiple (old) versions are live and for different reasons. But the fix is easy.
While working on this, I was wondering if this frozen node bug is also true for Generative Design.
And that it is fixed in Revit2021 and later versiosn.
We will continue testing this script. But I have good hope that it will work just fine.