There is (still) a problem with the headers sent when delivering the fonts for IE.
The cause seems to be that IE requires special headers to be set for the fonts and/or that they do not play nicely with redirection.
I found the following article:
http://www.unknownerror.org/Problem/index/875709962/ie9-css3111-font-face-encountered-unknown-error/
Actually, this error will probably solve itself when the delivery of asset will not use the "temporary solution".