Thanks Alexander! The stackoverflow thread and your explanation were very helpful and everything works now.
The python version does not throw any error. It's a silent error and only when looking at the number of faces I realized this issue exists. Maybe one could add a warning or exception in case a face was not correctly defined?
I agree with Andrew that having a tool to convert an unordered list of triangles containing vertex handles to an ordered list for face definitions would be very useful.
Thanks again and best regards, Marc