I’ve played around with the Windows port some more. It’s really weird. I get correct metadata for the loops, so communication with the library works. It’s possible to read just the metadata without creating a handle, but as soon as I try to actually create the handle, the REX library throws an error. It’s a generic (undefined) error, though, and dlerror() returns nothing - I have no idea how to debug that. I still think the two "NULL"s I pass are the problem. The library expects some stuff to set up a callback, and maybe the Windows version simply won’t work without a valid callback. The Mac version doesn’t seem to care, though.
Haha, glad to hear that! 
Still, “almost all files”? Could you post one of the files that won’t work? I’ll look into it. 