Did somebody create a new topic? Didn’t find one, also don’t know why this old one should not still fit? At least you find this one, if you search for some pattern related to the issue.
Whatever, I just faced the same, while having ownCloud installed just beside Nextcloud on Nginx. ownCloud Nginx example config still servers the X-Frame-Options header, thus as explained above the error shows up.
Removing the header solved the warning, BUT actually the header is NOT served, at least does not show up on curl request:
curl -v http://192.168.178.24/nextcloud * Trying 192.168.178.24... * TCP_NODELAY set * Connected to 192.168.178.24 (192.168.178.24) port 80 (#0) > GET /nextcloud HTTP/1.1 > Host: 192.168.178.24 > User-Agent: curl/7.52.1 > Accept: */* > < HTTP/1.1 302 Found < Server: nginx < Date: Sun, 19 Nov 2017 00:27:01 GMT < Content-Type: text/html; charset=UTF-8 < Transfer-Encoding: chunked < Connection: keep-alive < Location: /nextcloud/ < Strict-Transport-Security: max-age=15768000; includeSubDomains; preload; < X-Content-Type-Options: nosniff < X-XSS-Protection: 1; mode=block < X-Robots-Tag: none < X-Download-Options: noopen < X-Permitted-Cross-Domain-Policies: none < * Curl_http_done: called premature == 0 * Connection #0 to host 192.168.178.24 left intact
It is served if given on Nginx config, but then error on admin panel shows up. So this for me looks like an error on Nextcloud side.