Getting thousands of HTTP errors 509, but origin seems fine

So if Cloudflare returns s 523 it cant return a 509 at the same time. Probably its “converting” the 509 in your backend to a 523.
For error 523 please read here Community Tip - Fixing Error 523: Origin is unreachable or here

Lets check if the 509 error is comming from your origin, there are multiple ways for this:

  1. pause Cloudflare and call the Page again
  2. create a DNS entry which is :grey: and not :orange: and point him to your origin, then call it.
  3. use your local hostfile to set the domain to the real IP of your origin server/service and call it.

I personally recommend option 3 as it does not affect any other user, just your local machine. After you have done the test please report back if the origin directly responded with a 509 error.

Here some other posts about 509 Error, seems like it is indeed about Bandwidth/Ratelimit:

  1. error 509 - Too many connections on server
  2. 509 error for images
  3. Error 509 exceed bandwidth limit