Hi,
I believe it’s probably not related, as I don’t see any of the custom CSS conflicting with those icons. As I’ve looked at the source, it tells me that the content is simply ignored for those icons: http://i.imgur.com/lW53L7D.jpg
That info might be incorrect though, IE debugger tends to do that.
I’ve googled around, and there are actually known issues with font awesome and IE11. I have found these, which might help you:
– IE11 not showing font awesome icons
– Font-Awesome icons disappear in IE11 only after page refresh
– Font Awesome 4.2.0 not rendering in IE11 with Compatibility Mode turned On
– FontAwesome troubleshooting guide
I would first look at the version, to see if there is a possible update to that library, it may solve it. I have honestly no clue if this is actually the problem, I just suspect, based on the console information. Unfortunately explorer has a very limited debugger when it comes to fonts, so I was not able to check if it was correctly rendered or not.