Got HTTP status 404 Not Found for "http://john-chambers.us/~jc/music/book/cache/Tune-6449-air.abc/tmp"
pngcmd="(./ps2png 512 512 /Users/jc/w/tmp/Tune-92914-tmp.ps) >>/Users/jc/w/tmp/Tune-92914-get.log 2>&1" returned status 256
http://john-chambers.us/~jc/music/book/cache/Tune-6449-air.abc/tmp

Error exit status 256; here are some files to check:
[get.cgi V=1/1 B=0 scale=0.55 512x512 PNG]
filesizedescription
Coll_92914.log 263810 Log file, useful mostly for debugging
Tune-92914-get.log 3976 Log file, useful mostly for debugging
Tune-92914-tmp.abc 155 ABC music file with the extracted tune(s)
Tune-92914-tmp.ps 6 PostScript
Tune-92914-tmp.txt 155 Plain-text file with the extracted tune(s)
These files should be available for 24 hours.