What I think is happening here is that YouTube is periodically changing the algorithm it uses to encrypt video "signature" values...
[FYI - Youtube is now encrypting the "signature" values for certain channels and videos -- Vevo included.]
So if they keep switching up the cypher used to encrypt video signatures, it will make it hard to anticipate changes in the encryption algorithm. Especially if that algorithm changes every week.
So I need to look for a pattern in how they apply these changes to the encryption algorithm if we are to prevent this from happening over and over again. That is really the only sane approach here. Hopefully, there is some kind of pattern, and the cypher changes are not purely random.
That said, it is becoming clearer every passing day that YouTube does not want you to download certain videos, and they seem prepared to make it as hard as possible to do so.
Again, keep your fingers crossed, and I will continue to investigate solutions to this problem. And, as always, I will continue to post updates here.