I found this answer, which addresses the question more directly than the accepted answer, which really, shouldn’t have been the answer 🙂
And now to our main highlight – the “CSS3111: @font-face encountered
unknown error”. This error is very ambiguous. If you have a look at
MSDN again, you’ll see its description says: “An unknown problem was
encountered with the “Web Open Font Format (WOFF)”, and “Embedded
OpenType font (EOT)” of the Cascading Style Sheets (CSS) font”.
“Unknown Problem” doesn’t sound too good to me – how am I supposed to
solve an unknown problem? Fortunately we’re given a hint here. It
says: “Check source of the fonts”. Indeed, CSS3111 is usually caused
by an issue with the font’s binary source. One of the popular online
TTF to EOT converters for example produces EOT files with a NAME table
that doesn’t comply to the Microsoft standards, which results in EOT
fonts that never load in IE and produce the CSS3111 error. So, when
you experience a CSS3111, it is always good to try using a different
TTF to EOT converter or font face generator.
Source: http://www.marinbezhanov.com/web-development/16/how-to-embed-webfonts-properly-and-how-to-solve-the-ambiguous-css3111-font-face-encountered-unknown-error/