Got HTTP status 404 Not Found for "http://john-chambers.us/~jc/music/book/cache/Tune-24030-ThomasMarsden.abc/tmp"
pngcmd="(./ps2png 512 512 /Users/jc/w/tmp/Tune-19985-tmp.ps) >>/Users/jc/w/tmp/Tune-19985-get.log 2>&1" returned status 256
http://john-chambers.us/~jc/music/book/cache/Tune-24030-ThomasMarsden.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
Form_19985.log 833 Log file, useful mostly for debugging
Tune-19985-get.log 8632 Log file, useful mostly for debugging
Tune-19985-tmp.abc 166 ABC music file with the extracted tune(s)
Tune-19985-tmp.ps 6 PostScript
Tune-19985-tmp.txt 166 Plain-text file with the extracted tune(s)
These files should be available for 24 hours.