Userscripts.org

linkify youtube links are not linking correctly. They all direct to a screen that says

"The video you have requested is not available.

If you have recently uploaded this video, you may need to wait a few minutes for the video to process."

I noticed the same problem.
I found out, that all letters in the link get converted to lowercase letters.
Many URLS, however, are case senstive, and youtube video ids are, too.

I got it fixed by changing the line
a.setAttribute("href", href.toLowerCase());
to
a.setAttribute("href", href);