How to fix Nextcloud Refused to send form data to /login/v2/grant because it violates the following Content Security Policy directive: form-action 'self'

Problem:

When trying to connect using the Nextcloud client, during the Flow v2 authorization step where you open a page in the browser to authorize the client, you see an error message in the JS console like

Refused to send form data to 'http://nextcloud.mydomain.com/login/v2/grant' because it violates the following Content Security Policy directive: "form-action 'self'".

Solution

Add

'overwriteprotocol' => 'https',

after this line:

'version' => '18.0.0.10',

in your nextcloud/config/config.php.