Looks like it's complaining about the font. Perhaps try different values for the 'font' attribute, as described in the
spumux manual:
Code:
<subpictures>
<stream>
<textsub filename="demo1.srt" characterset="ISO8859-1"
fontsize="28.0" font="arial.ttf" horizontal-alignment="left"
vertical-alignment="bottom" left-margin="60" right-margin="60"
top-margin="20" bottom-margin="30" subtitle-fps="25"
movie-fps="25" movie-width="720" movie-height="574"
/>
</stream>
</subpictures>
I'm afraid I don't know much about subtitling. I've had lots of difficulty with spumux, and haven't managed to get anything other than menu-highlights (PNG subtitles, basically) to work.