Apple CDN connection error after changing server ip version to v6

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The universal links for my apps stopped working.

The server where the AASA files where hosted worked on IPV4 exclusively, a few days ago i changed the configuration to IPV6 only. I´ve created new IPV6 entries, renewed all certifactes and deleted all IPV4 entries for the domains. All seemed fine, but at Saturday I realized that my universal links stopped working for new user.

What i´ve done to find the issue:

Example domain that was used for debugging: "https://developffw.burns.fun"

I´ve verified the AASA file is hosted properly by using different browsers and Postman to retrieve it. The file can be accessed and the certificates look fine.
- Output of curl -v https://developffw.burns.fun/.well-known/apple-app-site-association

* Host developffw.burns.fun:443 was resolved.
* IPv6: 2a01:4f8:13b:340a::2
* IPv4: (none)
*   Trying [2a01:4f8:13b:340a::2]:443...
* schannel: disabled automatic use of client certificate
* ALPN: curl offers http/1.1
* ALPN: server accepted http/1.1
* Established connection to developffw.burns.fun (2a01:4f8:13b:340a::2 port 443) from 2a00:79c0:65c:8b00:80ee:175b:3e2a:1e7d port 61014
* using HTTP/1.x
> GET /.well-known/apple-app-site-association HTTP/1.1
> Host: developffw.burns.fun
> User-Agent: curl/8.16.0
> Accept: */*
>
* Request completely sent off
< HTTP/1.1 200 OK
< Server: nginx/1.22.1
< Date: Mon, 15 Dec 2025 11:34:22 GMT
< Content-Type: application/octet-stream
< Content-Length: 329
< Last-Modified: Sat, 21 Dec 2024 08:53:11 GMT
< Connection: keep-alive
< ETag: "676681f7-149"
< Accept-Ranges: bytes
<
{
  "applinks": {
      "details": [
           {
             "appIDs": [ "6LN7G8JEA5.burns.FFW-Manager-SwiftUI.Debug"],
             "components": [
                                 {
                                         "/": "/onboard",
                                         "?": { "id": "*"},
                                         "?": { "name": "*"},
                                         "?": { "token": "*" }
                                 }
             ]
           }
       ]
   }
}
* Connection #0 to host developffw.burns.fun:443 left intact

I took a look at the headers from the Apple CDN network response. These indicate some sort of connection error.
- The response code is 404
  - Response headers:

Apple-Failure-Details: {"cause":"dial tcp [2a01:4f8:13b:340a::2]:443: connect: network is unreachable"}
Apple-Failure-Reason: SWCERR00305 Network error
Apple-From: https://betaffw.burns.fun/.well-known/apple-app-site-association
Apple-Try-Direct: false
Via: https/1.1 defra2-vp-vst-003.ts.apple.com (acdn/268.16305), https/1.1 defra2-vp-vfe-004.ts.apple.com (acdn/268.16305), http/1.1 defra2-xdc-mx-028.ts.apple.com (acdn/3.16363), https/1.1 defra1-edge-fx-021.ts.apple.com (acdn/3.16363)
X-Cache: hit-stale, miss, hit-fresh, miss
CDNUUID: 4321f35e-b73b-4031-a054-7c63af69e126-712221049

Took a look at the log files of the server.
- I can´t find any entry from the Apple servers neither in the default logs nor in the error log entries.
- The curl attempts are logged with response code 200.
I´ve tried sudo swcutil dl -d https://developffw.burns.fun/onboard in the Terminal on my MAC.
- Output:

The operation couldn´t be completed. (SWCErrorDomain error 8.)

This indicates to me threre is an issue for the Apple servers accessing my server. But I don´t know what could be the reason. There is no firewall configuration that could block the requests. And there has been no change at all besides the IPV4 / IPV6 protocol change.

This issue is the same for all the domain listed on this server.

I´v even created a new app for this purpose and created a new AASA entry and associated link. Same issue.

I´m pretty much lost here. Everything looks fine from my side. Google assetlinks.json seem to work fine.

I would really appreciate some help on how to solve this, i´m at my wits end.

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