My Manga files were stored in a folder called "#Manga" (with a hash tag) and while I could see the squares and names, I couldn't see the actual cover.
Checking the terminal, I found the next error message:
qrc:/qml/GridComicsView.qml:232:13: QML Image: Cannot open: file:///path/to/#Manga/.yacreaderlibrary/covers/f9a59e9fd50ee38171f86043ce8d793ffed0b52e7811703.jpg
I could confirm this bug on both Windows and Linux (Ubuntu 16.04) machines.