My apologies if I should create a new thread instead of posting here, but this bug has appeared again. I'm using Kubuntu 19.04, and in a virtual box with the latest KDE Neon there aren't djvu thumbnails either. A related issue is that in maps with djvu files, Dolphin has problems with showing thumbnails of other files (like pdf's), unless I unclick the 'djvu-files' option in the preview-menu.

FWIW: I split the topic, as this is 2019, the bug was from 2013...

Please file a bug ob https://bugs.kde.org with detailed steps to reproduce.