When a major sub goes private, that's an awful lot of posts and comments that they have to hide from the rest of the internet. I'm not surprised there's some excess load on their systems with lots of them doing it at once.

Yeah, its one of the things that's always slightly ticked me off about that guide.

Excavating the history of the internet (mostly ye olde usenet groups) I couldn't find much he had to say about Sourcery, but there was this interview where he said:

By about book four, I discovered the joy of plot. (That was Mort.) I went back a bit with Sourcery [1988], because I knew the fans wanted more of Rincewind. I didn't particularly enjoy writing Sourcery, but it stayed on the best-seller list for three months. And then I said, "Sod the fans, I'll do what I like."

which doesn't make me think he'd particularly recommend it.

Yes. Publication order is definitely the best way to read the books. You get to see the world develop, You get to see Terry's skills develop. You get to follow Ridcully's and Stibbons' arcs in order. You get to spot all the crossovers and references of bigger events and minor characters.

But...

It can be a bit overwhelming for new readers if they're told that its a 41 book series, and the best character isn't introduced until book 8, and the best books start around book 19, and the first couple of books are in a bit of a different style to the later ones, and the best book is book 29.

So my generic suggestion is for new readers to pick up Guards! Guards! and then decide whether they're hooked enough to read the whole series in order, or want to carry on with the Watch books. If I know a bit more about the asker, I might suggest starting with a different one. If its winter, I'm likely to suggest Hogfather.

I've never seen a source on that suggestion to start with Sourcery. I'd certainly push people to a none-Rincewind story. Guards! Guards!, Wyrd Sisters or Small Gods.