After changing the MTU to 1200 I have no timeouts anymore, but a lot of read ECONNRESET errors. -.-

I just changed the Docker’s MTU to 1200 and will observe the change.

So the MTU of Tailscale is actually 1280, but is the connection even going through the VPN or rather through my VPS, when Uptime-Kuma is trying to connect to my local domain?

Thanks, since I access my home network and server through the public IPv4 of a VPS via Tailscale this could actually be the issue. I’ll look into it, when I find the time.

Not sure how this helps, but here you go.

Yeah, I suspect it’s simply an issue on the side of DuckDNS. :/

Yeah, it works fine through my browser. Sometimes the websites load a little longer. I feel like it’s an issue with DuckDNS as it’s seemingly random when it works and when not.

IPv6 doesn’t work:

docker exec -it Uptime-Kuma curl -6 proxmox.datenprolet.duckdns.org
curl: (6) Could not resolve host: proxmox.datenprolet.duckdns.org

Besides that the issue has disappeares since last night. I automatically restart all containers at night and moved from uptime-kuma:1 to uptime-kuma:latest. That shouldn’t make a difference, but maybe it did?

And it’s not a typo in my config, but in my post. But good catch. ;)

It’s HTTPS, what else should it be, when I monitor a domain?

What do you mean by tracker? I’m monitoring local domains, that point to local services and their respective web interfaces like Proxmox or Nextcloud. The local domains have a wildcard SSL certificate via DuckDNS.

Well, I’m monitoring the GUI of Proxmox on which I run a Debian VM which itself runs Uptime-Kuma and Nextcloud in Docker, so yes that’s on the same hardware.

Yep

Yes, Uptime-Kuma is running on the same domain as the other services, except the Nginx-Proxy-Manager, which runs on a VPS which I access via WireGuard. And yes, I’m using Docker. I tried curl’ing one of the domains from the Uptime-Kuma container and got the folllowing error: curl: (35) OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to service.datenprolet.duckdns.org:443. So thanks, now I have an idea about what I should investigate.

In Switzerland you get unlimited 10 Gbit/s for 50 bucks.