The https://help.nextcloud.com sub-site is running Nginx/1.10.0 which is vuln to a known issue (CVE-2016-4450) which allows a remote malformed HTTP request to cause the Nginx process to crash.
DoS testing is mentioned as not requested, but if you know of an issue give it a go …
You can determine the version running by requesting the IP of the site and getting the HTTP 301, eg: https://88.198.160.135
https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-4450