This issue has persisted since at least past June.
Attempting to access https://phabricator.wikimedia.org yields:
Internal Server Error The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrator at noc@wikimedia.org to inform them of the time this error occurred, and the actions you performed just before this error. More information about this error may be available in the server error log.
and in curl -IL:
HTTP/1.1 501 Not Implemented date: Wed, 04 Mar 2020 18:03:05 GMT server: Apache x-powered-by: PHP/7.3.14-1~deb10u1 x-frame-options: Deny content-security-policy: default-src https://phab.wmfusercontent.org; img-src https://phab.wmfusercontent.org data:; style-src https://phab.wmfusercontent.org 'unsafe-inline'; script-src https://phab.wmfusercontent.org; connect-src 'self'; frame-src 'self' https://commons.wikimedia.org; frame-ancestors 'none'; object-src 'none'; form-action 'self'; base-uri 'none' referrer-policy: no-referrer cache-control: no-store expires: Sat, 01 Jan 2000 00:00:00 GMT x-content-type-options: nosniff set-cookie: phsid=A%2Fp3j3qgmn2k4b2rss73s7gc6cvk6zxdn75iw77ufb; expires=Mon, 03-Mar-2025 18:03:06 GMT; Max-Age=157680000; path=/; domain=phabricator.wikimedia.org; secure; HttpOnly allow: content-type: text/html; charset=iso-8859-1 x-envoy-upstream-service-time: 99 X-ATS-Timestamp: 1583344986 Content-Encoding: gzip Vary: Accept-Encoding X-Varnish: 554083088 Age: 3 X-Cache: cp5009 miss, cp5012 pass X-Cache-Status: pass Server-Timing: cache;desc="pass" Strict-Transport-Security: max-age=106384710; includeSubDomains; preload Set-Cookie: WMF-Last-Access=04-Mar-2020;Path=/;HttpOnly;secure;Expires=Sun, 05 Apr 2020 12:00:00 GMT X-Client-IP: 45.247.231.22 Connection: keep-alive
Accessing the same address via a proxy works as expected.
Sometime in June 2019 I had sent a message to the email address cited in the error message, but got no answer.