In my limited XS experience, I've found it best to start putting in a lot of sv_dump() calls to wherever I feel suspicious in the XS source. I have not seen the code in question, so I can't offer you my opinions where that might be.

Since sv_dump() is not recursive (it won't follow references, nor parse array or hash elements), you may find Devel::Peek::Dump function more useful (if the code is in Perl, of course).