Please do not double post. See my reply to you in the Kobo forum.
And perhaps you could go the trouble of searching before posting? This issue was mentioned several times in the
[GUI Plugin] Kobo Utilities including error message identical to what you posted other than some language changes in the headers and in a couple of other threads.
If you use Google, try a search for HTTP Error 403: Forbidden site:mobileread.com just for the fun of it.