Got HTTP status 404 Not Found for "http://john-chambers.us/~jc/music/book/DftY/Button_Whitaker.abc/1818"
pngcmd="(./ps2png 512 512 /Users/jc/w/tmp/Tune-20845-1818.ps) >>/Users/jc/w/tmp/Tune-20845-get.log 2>&1" returned status 256
http://john-chambers.us/~jc/music/book/DftY/Button_Whitaker.abc/1818

Error exit status 256; here are some files to check:
[get.cgi V=1/1 B=0 scale=0.55 512x512 PNG]
filesizedescription
Tune-20845-1818.abc 157 ABC music file with the extracted tune(s)
Tune-20845-1818.ps 6 PostScript
Tune-20845-1818.txt 157 Plain-text file with the extracted tune(s)
Tune-20845-get.log 4005 Log file, useful mostly for debugging
These files should be available for 24 hours.