I just checked my forum error log and it is LOADED with this error...450 pages of errors since mid December. I see it happening with several different variations of the message. I've searched here and seen it happening for other people with some solved...but I don't see my particular problem. Can someone offer some assistance?
(http://www.woopig.net/steve_images/header_error.gif)
Try re-uploading Sources/Display.php and see if that fixes it.