I've just received a reply from Onyx Boox developers.
They say the device handles those new epubs as zero length books since images are now inserted not as elements, but as background.
4.6.5 page
Code:
<img src="../Images/zdeathnote-0004-0000-01-118-kcc.jpg" alt="zdeathnote-0004-0000-01-118-kcc.jpg" class="singlePage"
5.0.0 page
Code:
<body style="background-image: url('../Images/zzdeathnote-0004-0040-01-153-kcc.jpg'); background-color:#FFFFFF;">
Guess i'll stick to 4.6.5 for the time being.