volgens Luis de Vasconcelos.
Go to Site administration / Appearance / Themes / Theme (http://yoursite.com/theme/index.php) and click the Clear theme caches button. Does that "fix" the display?
If not, check your Moodledata permissions.
If those are ok then go to Site administration / Server / HTTP and turn off the Use slash arguments option. (NOTE that turning this option off might break scormactivities in your courses, so it's a workaround, not a real solution. And there's talk of removing that slash arguments option, so don't rely on it too much).
If that still doesn't help then have a look at the links listed in MDL-47645 for other things you can try.