The problem came from three things:
The default maximum size of files was set very low, 5 kB!
This maximum size was enforced even if the user didn't select to limit size.
* The maximum size limit was enforced before creating the list, giving the impression that AmiModRadio could not "see" certain files.
Now, the two first points are fixed and the third one required changing the format of the lists (and cached files) but now show a clear message when AmiModRadio encounter a too-large file.