Android App no longer connecting

We have a Nextcloud Hub 21.0.2 instance served by Caddy behind HAProxy performing reverse proxy and SSL termination.

All has been well, this has worked with ownCloud and Nextcloud and has served us well for many years. Unfortunately, and silently, the Android app on multiple phones and devices lost connection. Browsers on the same devices work on and off Wifi as well as on other platforms. I wish I could say what changed, I just can’t say WHEN it broke; I am assuming earlier this month it was working fine.

I’ve seen many forum hits on the subject, but nothing that I have found to offer up much hope.

Needless to say, I have many other web applications served by this same/similar implementation and this is the only one that is giving me fits and I have nothing meaningful in the logs on my server(s) that are helpful.

Talking to my future self

Check your IPv6 DNS records to ensure they are correct

I have the absolute same problem!
But here the mobile devices have an IPv6 DNS from outside my local network.

So you changed the IPv6 DNS inside the router? The mobile devices?

My IPv6 DNS record provided by my nameserver

The dyndns process was the culprit as it failed to update the DNS record. Which is why I didn’t see anything in my logs… It wasn’t reaching my webserver.

Ok,…
That I got this right:
You yourself in your local network have a local nameser which is for IPv6 too (e.g. to adress local devices by domains too)? And that thing was misconfigured?
OR
Was it the dyndns record for your domain (the AAAA record) that was misconfigured?

Because:
I only have this problem when the device is in my local network where the webserver is. When the device is outside the local network everything works fine.

Right, wrong, (or me being lazy)… it was the IPv6 dyndns record from my domain. However, because of the way I implemented, I could not access this whether LAN or WAN as the DNS record (AAAA record) had the incorrect public IPv6 address.

Than I got a different problem… The dyn AAAA record is managed by ddclient and so always correct.

My suggestion was the DNS in the network settings. But it doesnt make any difference. I could set the DNS to 1.1.1.1 or 9.9.9.9 or 8.8.8.8 or the router. In all the situations from my local network the nextcloud isn’t reachable by the app.

Any suggestions what could be wrong, too?

Though I’m pretty good at throwing darts, I’d suggest creating a new thread with your specific implementation details and symptoms to assist troubleshooting.