As it runs on my machine (!) one reason may be that right spelling should be : TableFormattingOptions.EXCEL_UNFORMATTED_TABLE
-hans-, which version of InDesign do you use? CS5.5? If so, did you install all the Updates there are for that version?
Okay, now it is working. I had to save the XLSX-file into the older XLS format, but that seems to work quite well.
But I won't ever understand why InDesign loses its backward compatibility from one version to the other.
Thank you, -hans-.