How does Minimserver build the upnp:AlbumArtURI property for content directories?

[simbun]

I'm afraid I don't have an answer, but I've just tried it on Windows with an 18 CD box set and it worked fine for me, the last AlbumArtURI was:

<upnp:albumArtURI dlna:profileID="JPEG_MED">http://192.168.0.12:9790/minimserver/*/master/2012*20-*20Blur*2021/CD18/folder.jpg</upnp:albumArtURI>

Have you tried putting a known working folder.jpg into the CD3 folder to see if it gets picked up (like the one from CD1)?
Are you having this problem with all box sets?