The code works when I add optode_frame = "mri" when importing the file! When plotting, it also looks like interpolation takes place. Thank you so much.
However, when looking at the provided link, I found the following text:
“MNE-Python only supports NIRx files recorded with NIRStar version 15.0 and above. MNE-Python supports reading data from NIRScout and NIRSport 1 devices.”
We didn’t use NIRStar but Aurora to record the data, which is the accompanying software to NIRSport 2 devices. I hope this is not a problem, though the text does imply that it is…