Comment 1 for bug 729848

I have the same problem too
Debug output

DVDStyler v1.8.1
Linux 2.6.35-28-generic x86_64
FFmpeg: libavformat 52.64.2, libavcodec 52.72.2, libavutil 50.15.1
Prepare
Cleaning temporary directory
Search for transcoded files in cache
Generating menus
Generating menu 1 of 3
Prepare
Create menu mpeg
Multiplexing subtitles (buttons) into mpeg
Executing command: spumux -s 0 "/tmp/dvd-tmp/menu1-0.mpg_spumux.xml"
DVDAuthor::spumux, version 0.6.18.
Build options: gnugetopt imagemagick iconv freetype fribidi
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INFO: Locale=en_CA.UTF-8
INFO: Converting filenames to UTF-8
STAT: 0:00:00.000
INFO: Picture /tmp/dvd-tmp/menu1-0.mpg_buttons.png had 1 colors
INFO: Picture /tmp/dvd-tmp/menu1-0.mpg_highlight.png had 2 colors
INFO: Picture /tmp/dvd-tmp/menu1-0.mpg_select.png had 2 colors
INFO: Pickbuttongroups, success with 1 groups, useimg=1
INFO: Found EOF in .sub file.
INFO: Max_sub_size=76
WARN: Button y coordinates are odd for button button02: 120x333-292x361; they may not display properly.
INFO: 1 subtitles added, 0 subtitles skipped, stream: 32, offset: 0.50
Statistics:
- Processed 0 subtitles.
- The longest display line had -1 characters.
- The maximum number of displayed lines was 0.
- The normal display height of the font arial.ttf was 0.
- The bottom display height of the font arial.ttf was 0.
- The biggest subtitle box had 76 bytes.
Generating menu 2 of 3
Prepare
Create menu mpeg
Multiplexing subtitles (buttons) into mpeg
Executing command: spumux -s 0 "/tmp/dvd-tmp/menu1-1.mpg_spumux.xml"
DVDAuthor::spumux, version 0.6.18.
Build options: gnugetopt imagemagick iconv freetype fribidi
Send bugs to <email address hidden>
INFO: Locale=en_CA.UTF-8
INFO: Converting filenames to UTF-8
STAT: 0:00:00.000
INFO: Picture /tmp/dvd-tmp/menu1-1.mpg_buttons.png had 4 colors
INFO: Picture /tmp/dvd-tmp/menu1-1.mpg_highlight.png had 5 colors
INFO: Picture /tmp/dvd-tmp/menu1-1.mpg_select.png had 5 colors
INFO: Pickbuttongroups, success with 2 groups, useimg=0
spumux: subgen-image.c:897: imgfix: Assertion `useimg' failed.
Failed