yeah, my vote also goes to: "non-specific half-elf afflliction, as yet incurable by conventional & magical healing methods Syndrome"*. Just like nearly all the other half-elves introduced in the comic so far, with the possible exception of Meji. (although her exaggerated behavorial instabilities could be a manifestation of that also).
I include Ian into that bunch, because prior to his encountering the Pedagogusi (sp), he showed similar signs, first seen here when Ian said his body was resisting certain types of magic:
http://www.errantstory.com/archive.php?date=2003-07-21 (and for that matter, Meji said that she has been having trouble with some similar magicks herself in the same strip).
Instead of trying to determine the specific medical causes of Leah's particular manifestation of the aforementioned "syndrome", why not just agree that to some degree, ALL half-elves are either affected from birth in some manner, OR have a very high rate of predetermination for it, and that the cause is due to some kind of genetic wonkiness between Elven and Human genes.
Granted, there are some incidental half-elven characters shown in parts of the comic (especially those regarding the history of the half-elves) where no visible physical manifestations are seen, but that doesn't exclude the possibility that they may have had some kind of psychological/emotional/mental abnormalities, nor does that exclude the possibility that there may be some cases where such a genetic abnormality may prove to be of benefit to an afflicted half-elf in some way (eg. ranging from possible resistance to various harmful ("black") magicks, to being able to "see" wavelengths of light extending beyond the normal visible light spectrum (ie. infrared & ultraviolet), to having a prehensile catgirl-style tail, for all we know.
Who knows? (well, Poe and Impy would, but you know what i mean)
- Ptrix
* Commonly known as "that half-elf thing"