album art not showed consistently

[ersy]

Hallo!
Yesterday I installed Minimserver2 on my QNAP-NAS. I browse with KAZOO and use an Ipad.
Installation was in so far successful that I can see the Minim icon on my QNAP screen but when I try to open it nothing is shown from my data, folders etc.
When I do open the Twonky icon I see immediately all my audiofiles etc.
So failure somewhere!?
Last night I got a headphone session and curious I decided to click back to the Twonky level. To my big surprise I saw there, alongside the Twonky icon, also the Minim icon!
I clicked on it and everything was there! Choice possibilities for album, artist, folder etc! Hurray!

But my problem is the way that album art is showed/not showed.
When e.g. I open “Albums” the file opens and every album shows its own albumart. So I'm shure it really is all there!
But if I open “Folder” or “Artist” only 1 or 2% from all the files shows its album-art. The rest shows the music note icon
Opening a file and again all album-art is there … complete!
Hope you have a suggestion what to do.