This article is about Cloudflare Rocket loaders passion to break Nextcloud (and apparently PhpMyAdmin for some people) and how to fix it.
I have installed Nextlcoud for some testing purposes some time ago. As a side note; Yes, I’m using Cloudflare for DNS. And was surprised by Nextcloud’s clean UI, consisting of only the logo and a background image.
Go into your domains Cloudflare setting, then choose Speed and go to “Optimization”, disable the Rocket Loader™ option. Your service should start working properly again.