Log in / sign in button does not work

Steps to reproduce
1. go to owncloud server site
2. enter login credentials
3. press 'enter' button or click on arrow to login

Expected behaviour
Logs in and shows users data

Actual behaviour
It's like enter button / arrow was not even clicked.
Not even a refresh

Server configuration
Operating system: Debian
Web server: Nginx
Database: MySQL
PHP version: 5.6.0
ownCloud version (see ownCloud admin page): 9
Updated from an older ownCloud or fresh install: updated from 8
Special configuration (external storage, external authentication, reverse proxy, server-side-encryption): none

ownCloud log (data/owncloud.log)
owncloud.log has 2 GB

Please paste possible errors in the following code block, see https://central.owncloud.org/t/how-to-find-webserver-or-oc-logfile-enable-php-logfile/808 for more info

Integrity status for oC9+

Login as admin user into your ownCloud and access
paste the results here.

This is known to be caused by e.g. Cloudflare and other services or webserver modules messing around with the javascript used on the login page.

Thank you.
If anyone's interested, CloudFlare DNS was set to 'DNS and HTTP proxy'.
I changed it to 'DNS only'.