OPTION 1. Before you launch your browser, you should open a book and change the font once.
OPTION 2. if OPTION1 doesn't work, you can delete the gothic mb101.otf & ruymin.otf (they are japanese fonts) via the attached KoboRoot.tgz.( just extract the attachment and put the KoboRoot.tgz in the .kobo directory), than try OPTION1 again.
caution:After this operation, you can't read japanese books, but you can just put a japanese font in fonts directory to fix it.
extractMe.zip
Finally I want to say the kobo's browser is some stupid and not worth to use.