In my case it appears to be browser related. I got that error on desktop Firefox but tried with mobile safari later and it worked

That's interesting. Could that be the reason why Instapaper doesn't retrieve and save archive.{etc} pages? Is Instapaper using Cloudflare DNS to do so? Pocket or Raindrop.io for example have no problems with archive.{etc}.

I'll check that out in the morning, thanks. I think I changed my network DNS for troubleshooting and forgot to change it back.