CloudFlare + Azure VM with IIS WCF Service returns 520 sometimes

I have setup an Azure VM having IIS. I mount a WCF Service in the domain and setup the SSL there.
On CloudFlare I set SSL to be Full.

When I try to see the service URL on browser works awesome but if keep refreshing the browser then randmonly I get 520 error. It doesn’t have a pattern to follow, it can happen after 25 refreshses, or 10 or 30, etc.

I then created a Page Rule to disable Cache and also disable Always ON and I still see the same behavior.

Then what I did was to look into IIS logs, keep record of last log date and time and then start refreshing the browser until I got 520. Then went again to see the logs and all the requests are logged there with 200 so the server is not the problem. Actually If I remove CloudFlare completly and browse the service using the server IP address I never never get any issue. Also on Azure I setup Insights and there is no exceptions, timeouts, anything that could lead to a cause.

Any clue on what I can do? Is there a way to see in CloudFlare portal all the logs (origin request and responses)?

As extra note, if I pause CloudFlare i never see this issue, of course i have to test using HTTP instead of HTTPS, but never see a 520. This only happens when the URL has HTTPS. So I dont know if this could be a SSL Handshake problem that is intermittent because something, is really breaking our heads.

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