Here is the GET request, like you can see, even if HTTP 200, I get an empty content AND a warning.
Might be a duplicate of T86611... but quite unsure about that.
$ curl -v "https://en.wikibooks.org/w/api.php?action=visualeditor&format=json&paction=parse&page=Calculus%2FPrint_version&oldid=3101846" * Hostname was NOT found in DNS cache * Trying 208.80.154.224... * Connected to en.wikibooks.org (208.80.154.224) port 443 (#0) * successfully set certificate verify locations: * CAfile: none CApath: /etc/ssl/certs * SSLv3, TLS handshake, Client hello (1): * SSLv3, TLS handshake, Server hello (2): * SSLv3, TLS handshake, CERT (11): * SSLv3, TLS handshake, Server key exchange (12): * SSLv3, TLS handshake, Server finished (14): * SSLv3, TLS handshake, Client key exchange (16): * SSLv3, TLS change cipher, Client hello (1): * SSLv3, TLS handshake, Finished (20): * SSLv3, TLS change cipher, Client hello (1): * SSLv3, TLS handshake, Finished (20): * SSL connection using ECDHE-ECDSA-AES256-GCM-SHA384 * Server certificate: * subject: C=US; ST=California; L=San Francisco; O=Wikimedia Foundation, Inc.; CN=*.wikipedia.org * start date: 2017-11-03 03:42:02 GMT * expire date: 2018-11-22 07:59:59 GMT * subjectAltName: en.wikibooks.org matched * issuer: C=BE; O=GlobalSign nv-sa; CN=GlobalSign Organization Validation CA - SHA256 - G2 * SSL certificate verify ok. > GET /w/api.php?action=visualeditor&format=json&paction=parse&page=Calculus%2FPrint_version&oldid=3101846 HTTP/1.1 > User-Agent: curl/7.35.0 > Host: en.wikibooks.org > Accept: */* > < HTTP/1.1 200 OK < Date: Sat, 31 Mar 2018 16:20:48 GMT < Content-Type: application/json; charset=utf-8 < Transfer-Encoding: chunked < Connection: keep-alive * Server mw1344.eqiad.wmnet is not blacklisted < Server: mw1344.eqiad.wmnet < X-Powered-By: HHVM/3.18.6-dev < X-Content-Type-Options: nosniff < Cache-control: private, must-revalidate, max-age=0 < P3P: CP="This is not a P3P policy! See https://en.wikibooks.org/wiki/Special:CentralAutoLogin/P3P for more info." < X-Frame-Options: DENY < Content-Disposition: inline; filename=api-result.json < Vary: Accept-Encoding < Backend-Timing: D=2089499 t=1522513246467289 < X-Varnish: 434122591, 549146762 < Via: 1.1 varnish (Varnish/5.1), 1.1 varnish (Varnish/5.1) < Accept-Ranges: bytes < Age: 0 < X-Cache: cp1065 pass, cp1065 pass < X-Cache-Status: pass < Strict-Transport-Security: max-age=106384710; includeSubDomains; preload < Set-Cookie: WMF-Last-Access=31-Mar-2018;Path=/;HttpOnly;secure;Expires=Wed, 02 May 2018 12:00:00 GMT < Set-Cookie: WMF-Last-Access-Global=31-Mar-2018;Path=/;Domain=.wikibooks.org;HttpOnly;secure;Expires=Wed, 02 May 2018 12:00:00 GMT < X-Analytics: ns=0;page_id=52962;https=1;nocookies=1 < X-Client-IP: 10.68.16.224 < Set-Cookie: GeoIP=:::::v4; Path=/; secure; Domain=.wikibooks.org < * Connection #0 to host en.wikibooks.org left intact {"warnings":{"result":{"*":"This result was truncated because it would otherwise be larger than the limit of 12,582,912 bytes."}}}