Exporting with subtitles isn't working if there is a whitespace in the path or filename. I tried to fix it, but I can't seem to figure out how to escape whitespaces in the subtitles= param. Wrapping it in single or double quotes doesn't work. Escaping it with "" or "^" doesn't work either. I've also tried replacing spaces with "%20".
Edit: A workaround I've found is to extract the subtitles into a srt file, then use that file in the subtitles= param.