We were also experiencing this issue on an individual machine running IE. By using the developer tools in IE, I was able to see what was going on in our case:
- The webresource request that provides the css for the radeditor was cached in IE and no matter what I did to clear the cache, it always returned a 304 from the server and wasn't updated.
- The cached css was referencing another webresource URL for the sprite image that is used for all of the button icons on the radeditor. That webresource URL was returning a 404.
In the end, I lowered the FrontendResourcesCacheDuration setting under Administration->Settings->Advanced->Appearance in an attempt to stop the server from always returning a 304 on the css request. After restarting everything and making sure the timeout had passed, we refreshed the page. The CSS was refreshed from the server and it gave us a new WebResource URL for the sprite image that was valid instead of a 404. The icons are now all displaying happily.
I don't know how some webresource links were expired while others were still listed as valid, but I don't care now that I've solved my immediate problem.
I've now returned the FrontendResourcesCacheDuration setting back up to it's original value and the site is back to running normally.