I meant the options of your margin within your kobo device, when you call the menu Aa.
Also, the only difference with your book and mine, is that I remove the -moz-hyphen attribute in the book’s body CSS. So I suspect the kobo renderer is mixing up two algorithms for hyphenate the word. I got the case in french some time ago.
What you could do is :
1. to remove the hyphen inside the CSS. To doing this, you select the book in calibre, right clic, Edit the book, scroll to the CSS file like in
this picture, remove the body part, save the book. I’m pretty sure the auto value is the cause. It was, in my case, when I tried with french.
2. remove the book from kobo via kobo
3. plug the kobo to the computer
4. send the modify book back
5. check for the problem
6. give me the position of your slider (aA menu of kobo when inside a book) so I can get the book in the exact same look and feel that you. It happen I got my hand on a Kobo Glo HD this morning for update. The update (and replacement of the SD card) goes well so I can test it on the same device than you.
Waiting for your feedback
(EDIT : If you have problem with the 1. , I can send your book back to you (and eventually do a video to show you how to remove these specific lines automatically with a calibre plugin later this evening)