Thanks for your reply!
I’ve modified functions.php, as you suggested, but the problem still occurs:(
Do you have any other ideas?

I’m able to reproduce that problem on my computer by installing the limited version (lacking the latin-ext subset) of the font in my system (windows 7):
https://www.dropbox.com/s/386auuoooukephf/OpenSans-Regular-webfont.ttf?dl=0

With the .ttf file installed in C:\Windows\Fonts directory, some browsers seem to prefer it over the webfont provided in css. As you can see the header is not displayed correctly in IE and Firefox:
http://oi61.tinypic.com/2qiwvoh.jpg