Argo tunnel blocking some clients?

Some of my clients have been reporting being unable to access our services. As an example this page : https://api.explor.ai/ping is a web service hosted behind cloudflare (connected through argo tunnel). This particular page simply returns the text “pong”.

Some clients (now including me since at least mid-day today), have been reporting receiving the standard “Error 522” page from cloudflare. Consistently (no sporadic, they just cannot access the service).

Just as my clients, it is now impossible for me to visit my service from home. I tried many peripherals at home, and different browsers, they all give the same error. It seems to be related to a specific internet configuration somehow (the tunnel works correctly if I use my phone to visit it or a service such as wheregoes.com). I did reset the router/modem twice, and the only specific configuration I made in it was to use 1.1.1.3 DNS … but this configuration has been there for months… Cloudflare reports no outages, so I am completely lost here…what could be happening?

I have been making some progress, and it seems to be related to key inserted by Cloudflare on the headers/cookies. I have been able to reproduce the error with Postman. First request, without a cookie, goes through. Second request does not, and the “domain” in the cookie looks odd (with “.explor.ai” instead of “api.explor.ai”) :

__cfduid=d11ebdd51d39af5f7b14ceb478c8855701602339326; Path=/; Domain=.explor.ai; Secure; HttpOnly; Expires=Mon, 09 Nov 2020 14:15:26 GMT;

When that cookie is present on client device, the request never reaches the server.

Also, replacing the __cfduid= with “” (empty string) makes every request work again. This really look like a problem on Cloudflare side…