We have an issue when protecting one of our web-based endpoints today with Access. So the domain and URL is proxied over CloudFlare and work fine. SSL/TLS is already set to Full and it has been working fine always.
We were trying to enable Access on the admin area (e.g. domain.com/folder/admin) when this is enabled, trying to access that URL gives a ERR_TOO_MANY_REDIRECTS issue. Removing the application from Access resolves the issue.