IE 11: error CSS3111 in my own code, and google.com/fonts doesn't render any fonts

Question

I am developing a website that uses the Google font Open Sans like so:

<link href='https://fonts.googleapis.com/css?family=Open+Sans:400,300,300italic,400italic,600,600italic,700italic,800italic,800,700' rel='stylesheet' type='text/css'> 

Normally, I use Chrome when working with my website, but today I decided to see how it looks in IE 11 (11.0.10240.16431) on Windows 10. Unfortunately, Open Sans isn't being loaded and rendered properly. I see lots of these errors in the Developer Tools console:

CSS3111: @font-face encountered unknown error. PRmiXeptR36kaC0GEAetxjqR_3kx9_hJXbbyU8S6IN0.woff 

Thinking that was strange--I had previously developed a site that loaded Google Fonts just fine in IE 10--I headed on over to https://www.google.com/fonts. More CSS3111 errors, with every custom font being displayed in serif instead:

Is Google Fonts simply broken for IE 11? The fonts do load correctly in Edge, Chrome, Firefox, etc. I am at a loss for how to proceed to get these fonts to work in IE.

UPDATE 1

Setting the emulated document mode to 8 in IE 11 causes the fonts to render correctly. IE 9+ still exhibited the same issues, however. Is this some kind of incorrect user agent string processing by Google, perhaps?

UPDATE 2

I went to FontSquirrel and downloaded Open Sans in all its formats. I also imported the CSS provided in the ZIP. Unfortunately, IE and now Firefox continue to report that the font can't be used. Firefox says downloadable font: not usable by platform.

UPDATE 3

I've confirmed that IE's Font download setting is set to Enabled for all security zones.

Answer 1

For me, this issue was caused by a Windows 10 feature called Untrusted Font Blocking. My office network had this turned on in our group policy settings.

Using this feature, you can turn on a global setting that stops users from loading untrusted fonts that are processed by the Graphics Device Interface (GDI). Untrusted fonts are any fonts that are installed outside the %windir%/Fonts directory. https://support.microsoft.com/en-us/kb/3053676

To disable Untrusted Font Blocking using Group Policy:

1. Open Group Policy Management Editor
2. Under Local Computer Policy, expand Computer Configuration, expand Administrative Templates, expand System, and then click Mitigation Options.
3. In the Untrusted Font Blocking setting select Do not block untrusted fonts

To disable Untrusted Font Blocking using Registry Editor:

1. Open Registry Editor (regedit.exe) and go to the following registry subkey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Kernel\
2. If the MitigationOptions key is not there, right-click and add a new QWORD (64-bit) Value, naming it as MitigationOptions.
3. To turn this feature off. Type 2000000000000.

IMPORTANT: A computer restart is required for the changes to take effect