I looked at this really quickly, it wasn't my first 2 guesses, but
here's a workaround for which I can't immediately say why it works (and
something else odd might also be going on with that site)...
The cause seems to be that the site is extremely sensitive to the HTTP
request headers (*not* the usual anti-bot/anti-abuse User-Agent header
alone, nor the usual blocking of Tor US exit node IP addresses).
This worked for me (copying headers that worked in Tor Browser, using
the Firefox Web developer tools that you usually get by pressing F12); I
have not narrowed it down further:
(get-pure-port (string->url url)
(list "User-Agent: Mozilla/5.0 (Windows NT
6.1; rv:60.0) Gecko/20100101 Firefox/60.0"
"Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"
"Accept-Language: en-US,en;q=0.5"
"Accept-Encoding: gzip, deflate, br"
"Connection: keep-alive"
"Upgrade-Insecure-Requests: 1"
"Cache-Control: max-age=0"))))
The reason for the extreme header sensitivity might be anti-bot/abuse
fingerprinting, or a strange Web programming bug, or some buggy
server-side proxying/'firewall'/routing, or something else.
(BTW, something else odd might be going on with that site. Specifically,
on a successful request, the Firefox built-in developer tools see only
the one request and response (no redirect), of MIME type `text/plain`,
*but* my custom uBlock Origin ruleset is blocking two additional
requests, for scripts and fonts, which I wouldn't expect to be attempted
at all from a single request and `text/plain` response (maybe it's a
favico request?). This is probably nothing, and I don't have time to
investigate it right now, but a super-vigilant cybersecurity person
couldn't help but start to consider threat models for users of that site
(e.g., government users, big/strategic companies, activists). If you
were curious, debugging HTTPS traffic is a pain when you're not sure you
can trust the browser, but one could copy the key from the browser, and
capture the traffic with something like Wireshark or tcpdump, and see
what's going on. I kinda regret visiting it, and will try to forget it. :)
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