Found the bug!
One bug is due to inaccurate instructions: “… *Prepend the list [ None ] in front of the list of input characters. …”
As I understood it, in the definition of X the None should be prepend to the list of characters. Your print statements made me realise that it should be prepend to the list of indices.
The second bug was that I defined Y to be slice + append at the same time: X[1:].append(ix_newline).
Apparently these should be separated in order to work: first assign the slice to Y, then Y.append(…).
Thank you for helping!