[GeoffR]

Some more testing (I've made a Dutch ebook that contains nothing but "onderwerp" followed by various punctuation) and have found some interesting things:

1. The Dutch hyphenation dictionary supplied with the Kobo firmware is incorrectly encoded: It is marked as UTF-8 but it is actually encoded as ISO8859-1. However converting the file to UTF-8 didn't fix the onderwerp problem. It could be that the hyphenation dictionary needs to be rebuilt from scratch as a UTF-8 dictionary?

2. The bad hephenation of onderwerp occurs in the EPUB reader as well as the KEPUB reader. I've never seen this type of problem in the EPUB reader with the English hyphenation dictionary in over two years of reading epubs, so I suspect the real problem is the Dutch hyphenation dictionary.

Edit: I find can add rules such as 4p, to the dictionary to prevent the bad hyphenation in the EPUB reader, but it doesn't work for the KEPUB reader. So as well a problem with the hyphenation dictionary, there are still problems with the KEPUB hyphenation algorithm too.