yup I got the same error like you.
It is not error from our application, it is error from Fly infrastructure. I think the problem is described in this topic:
In the log I got same error:
could not find a good candidate within 90 attempts at load balancing. last error: no known healthy instances found for route tcp/443. (hint: is your app shut down? is there an ongoing deployment with a volume or are you using the 'immediate' strategy? have your app's instances all reached their hard limit?)
In this case we can not do anything except destroy machine, builder and recreate it to redeploy again