Got HTTP status 404 Not Found for "http://john-chambers.us/~jc/music/book/DashingWhiteSergeant.abc/500"
pngcmd="(./ps2png 512 512 /Users/jc/w/tmp/Tune-24038-500.ps) >>/Users/jc/w/tmp/Tune-24038-get.log 2>&1" returned status 256
http://john-chambers.us/~jc/music/book/DashingWhiteSergeant.abc/500

Error exit status 256; here are some files to check:
[P=get.cgi V=1/1 B=0 scale=0.55 512x512 PNG]
filesizedescription
Tune-24038-500.abc 156 ABC music file with the extracted tune(s)
Tune-24038-500.ps 6 PostScript
Tune-24038-500.txt 156 Plain-text file with the extracted tune(s)
Tune-24038-get.log 4027 Log file, useful mostly for debugging
These files should be available for 24 hours.