Out of curiosity do you use a VPN? I get issues like this a lot when browsing via VPN

Was there any feedback on the reason or mechanism for blocking?

I got blocked too. Got the following message:

  Blocked from Searching or Viewing the Ad Archive

  You have been temporarily blocked from searching or viewing the Ad Archive due to too many requests. Please try again later.

Is it really an "archive" if you can't download the contents at a reasonable rate? (Eg: I wouldn't expect gigabit speeds, but if you're willing to go slow and steady a 500k/1meg a second that should be doable)

That would enforce actual transparency. We can't allow that to happen! Here's how it works: You tells us who you are and what you're looking for, then we'll see what we can show you and what we can't.