Hi, I’m the author and also a proxy user like yourself. That guide doesn’t cover use behind a proxy, so here’s what you need to ensure is in your config.php file:

'overwrite.cli.url' => 'https://cloud.myserver.com',
'overwriteprotocol' => 'https',

This forces NC to use HTTPS despite not being configured that way.

What you’re experiencing there is totally normal behaviour as you’re proxying from an SSL connection to plain HTTP. Nextcloud will as such respond in plain HTTP unless you tell it otherwise.

Be aware, if you access internally via IP over HTTP, this will either a) give you SSL errors or b) not work at all (443 is not configured by default, so it may refuse to connect when it forces, due to that setting, the connection from http:// to https://)