It occurs appears when I try to use a SSL client certificate with Safari. Of course, authentication is broken as it just fails on an 403 error page.
So it seems that Safari is the last browser which was not patched against CVE-2009-3555 !
2009 !! At least, I quickly checked the other browsers I had around and they were fine: IE, Firefox, Chrome… I am having an issue with Opera also, but although I have not identified the problem yet, it seems unrelated (and does not throw the same error).
Note that I reported the issue to Apple, but I did not receive any answer. Silence on the wire.