the zip file has a full path embedded in it.
Any chance you can remove the full path ?
replace var/www/mellberg/FirePlay.swf? [y]es, [n]o, [A]ll, [N]one, [r]ename: n
I now get a blank page rendered with this version as well.
I noticed this in my logs
Thread 1662: Error opening /opt/share/mt-daapd/admin-root/FirePlay.swf: Permission denied
Mt-daapd runs as guest, so had to chmod FirePlay.swf to allow guest access.