Lots of people are getting the notorious "We're sorry, this video is no longer available" when tyring to access YouTube videos that do in fact exist. This is not a floatbox issue but rather a problem with the YouTube service. It's been happening frequently and unpredictably to people for about half a year now. The problem can occur even when viewing videos directly from the YouTube site pages. (Search on that error message text to see the extent of it.)

One day, YouTube stopped working in both IE6 and IE7 for me. Here's what fixed it for me.

From the IE menu, Tools / Internet Options / Connections / LAN Settings - make sure "Automatically detect settings" is cleared (not checked). Many other people have reported success after uninstalling the Google Web Accelerator.

This is all well and good to fix your own browser's behaviour, but it doesn't help us much in delivering YouTube-hosted videos to our site's visitors. You may want to consider finding one of the many YouTube video grabber utilities that are out there and copying the YouTube flash file to your own server until the problem is fixed.