PocketBook Era : manga covers not showing in Library view

I have a problem with 3 mangas (from the same series), bought from amazon, converted via Calibre to zip, then CBZ, then sent to my PocketBook Era : no matter what I try, I just can't get the cover thumbnails to show. I've tried manually renaming the zip to CBZ, I've tried the Embed Comic Metadata Plugin, I've manually renamed cover.jpg to 00000.jpeg/00001.jpeg and added it to the images in the zip, I've tried the experimental "embed cover" in the Embed Comic Metadata plugin, I've tried downloading the covers from the internet and manually setting them in Calibre, I've tried just about anything I can think of, the thumbnail pictures won't show.

It does work with every other manga I have (with exactly the same process : bought on amazon - some DRM magic - Convert to Zip - Convert to CBZ - Sent to the PocketBook Era via Calibre), and it does work if I convert to epub, and not to CBZ.

But if I take the (already converted, with-thumbnail-showing) ePub, and convert it to zip, then CBZ, either manually or through the Embed Comic Metadata plugin.... it doesn't work, the thumbnails don't show.

Moreover, the covers exist, since they show up as lockscreen/sleepscreen either with the stock reader or with Koreader.

I'm guessing there is some thumbnail database somewhere which isn't being updated, but I have no idea where....

Does someone have an idea ? Thank you !