This is more of a JSYK
There seems to be a relation to the size of the list but it wasn’t happening on macos 10.13.
VOX stops responding after clicking Save on large playlists lists, relative paths ticked or not doesn’t make a difference.
XSPF saving works fine. The largest successful save (I keep saving when making changes to my library and its .m3u for compatibility with another software) I managed to make was 10431 lines (file lines, not songs as I notice VOX adds some metadata as well as comments)