I'm currently running a medium sized Multisite install on an Ubuntu 12.04 x64 based server, using Nginx, PHP-FPM, MySQL and Cloudflare at the front, serving wp-login and wp-admin over HTTPS. Cloudflare isn't caching any wp-admin/login page, rules are in place, neither is the backend Nginx cache I have in place, both are showing as BYPASS in the headers.
It's mainly for hosting educational blogs for schools, who unfortunately use IE most of the time.
I've been alerted today that users can't log in to the admin area if they're using any variation of Internet Explorer, but all other browsers work without issue. Being a Mac user I rarely use Internet Explorer if I can help it, but when testing it in various scenarios I get exactly the same result: "ERROR: Cookies are blocked or not supported by your browser. You must enable cookies to use WordPress." This is on a proxied connection in a school, a direct connect at home and tethered on my phone's connection, so it's not an issue with proxying.
Looking in the developer tools on IE, the cookies are sent and received fine, exactly how they are in every other browser. The test cookie is generated etc, but on POST the page will just refresh with the same cookies, but then show the message about cookies being blocked as above.
I have tried all the common fixes available on Google and these forums, including modifying wp-login.php to force a login, functions.php code to force creating a cookie with the cookie-domain and setting the cookie defines in my wp-config.php. None of these have worked, but I didn't expect them to as it was already working in other browsers. I've yet to disable all the plugins that are network enabled because it is a live Multisite install, but I've tried quickly disabling some that could be the culprit, I'll try the rest during the night when it's quiet. Stranger still, I have some blogs hosted that use domain mapping and it will allow me to log in to those blogs in IE.
Has anyone witnessed this before? I have had the message before on all browsers when there was an error with a plugin at one point in the past, but this has since been fixed.