This happens when you have mixed content references on your page. When you access a website via https:// you can not have any links or references to http:// within your site. 

as an example, if you're using a reverse proxy (i.e. nginx proxy manager) and you proxy it to odoo via http:// not enabling the rewrite module could result in this issue.

Saying that, with the cloudflare tunnel there is an option in SSL/TLS -> Edge Certificates -> Automatic HTTPS Rewrites which will help fix mixed content. Try enabling this and see if this fixes it.

I would check to see if this is enabled on your service.