A generic 403 sounds like it’s your server that’s blocking access, at least by the screenshot you’ve shared. If you’re seeing a 403 error without Cloudflare branding, this is always returned directly from the web server, not Cloudflare, and is generally related to permission rules on your server.
I’d suggest you to temporary use the “Pause Cloudflare for this site” option, which can be found at the bottom right corner on the Overview screen of the Cloudflare dashboard.
Therefrom, make sure your site works fine over the HTTPS without errors and only then unpause/activate Cloudflare.
May I ask what kind of the suggested steps for troubleshooting have you tired already as written on the article from below? 
Are you still experiencing this issue?