Reply from Allen:

The problem is fixed. I have mentioned before that the MP3/headset mode is automatically used by a program to download the MP3 files back to our server. However, YouTube doesn't like programming bots like this, and from time to time, it will impose measures to block this. Every time this happens, I have to amend the program to circumvent it, and this becomes a cat-and-mouse game. I can't guarantee that I can always come up with a solution, so please expect that one day, I may not be able to fix this, forever.

Of course, we can ask our staff to manually render the MP3 and manually upload it to the server, but frankly, we don't have the extra resources to do that.

I deeply regret introducing this function because, on day one, we didn’t have it. But once I implemented it, out of courtesy, there is no way we can withdraw it, even though there are technical difficulties that we can't overcome, and people don't understand the effort I have put behind it.