same problem here
nginx 1.14.0 & php7.0 & rc 1.3.6
only happens with roundcube
nginx error: "2018/05/17 03:11:38 [error] 30583#0: *7 flv live: HTTP method was not "GET" or "HEAD", client: 91.XX.XX.XX, server: webmail.url.tld, request: "POST /?_task=login HTTP/1.1", host: "webmail.url.tld", referrer: "h$"
so i think i need to change ./ to ./index.php - but i dont know where
or where to set index.php as defalt