Hi,
Do those errors appear occasionally, or constantly? Only it’s possible some bot happened to make requests to the app at the same time and did not include any host (as the error message reports).
Take a look at this thread for some more on that particular issue.
If that error is logged for every request (including ones you are making e.g from a browser, and so do include a host header), the other possibility relates to using force_https. I don’t know if you are using that option, or any proxy (such as Cloudflare)?
In my case I had to manually restart my app using fly machine restart ID to get it going again after Fly migrated it.