Filenames starting with digits are no problem, but ids cannot start with digits. So, if you (or something else) create the id value from the filename, you have to add something else:
This is wrong:
Code:
<item id="01" href="01.html" media-type="application/xhtml+xml" />
This is right:
Code:
<item id="file_01" href="01.html" media-type="application/xhtml+xml" />
Note that the filename in href does not change.