If your app itself is healthy (fly checks list, fly status --all and fly logs are a great jumping off point if you’re not sure yet), then I wonder if the Cloudflare proxy settings are pointed to the correct IPv4/IPv6 addresses.
Specifically, if this app has a shared IPv4, then I wouldn’t expect Cloudflare to be able to connect to it at the IP level.
If this is the case, and you need IPv4, then you can allocate a dedicated IPv4 address, which might be worth trying to see if this clears things up.
- Public Network Services · Fly Docs
- Announcement: Shared Anycast IPv4 - #3 by kurt
- Fly App Pricing · Fly Docs
Alternately, depending on your usecase for Cloudflare proxying, you could turn off proxying to the app as a test, too.