Because I search on google about this url and it seems is related to Lets Encrypt, I am supposed to redirect this url to index or just ignored it? Or what I am supposed to do with it?
It’s fine. You don’t need to do anything with it.
http://www.example.com/.well-known/acme-challenge/check-your-website-dot-server-daten-dot-de URLs are used by https://check-your-website.server-daten.de/, which is run by one of the forum members here, to test how your web server responds to a request for an ACME challenge URL that doesn’t exist.
Your website might redirect the request to HTTPS. That’s probably fine.
If your ACME client is successfully able to renew your certificates, that’s all that matters.
If you want the 404 errors to go away, you could put /.well-known/acme-challenge/ in your robots.txt file so that Googlebot won’t try to visit it anymore, but it’s up to you. It doesn’t matter to Let’s Encrypt.
The tool is public (and free), all checked domains are public, all informations found are public.
So there is no "hidden secret". "Security by obscurity" doesn't work.
And the tool has a lot of Google-Traffic, nearly completely to the result pages. I don't want to stop that Google-Traffic.
The older results (?i=random-string) have a canonical attribute, so Google lists only the newest result (q=domainname).
If someone wants to hide informations, he shouldn't have a domain and he shouldn't ask in public forums. It's the same reason why this forum is public and has Google traffic.
If someone doesn't want that, he shouldn't consume resources of other people. Instead: Buy a certificate and hire a freelancer. Then it's secret.
Asking in a forum -> checks are required to answer -> the domain name and tools like letsdebug / ssllabs / check-your-website are required to answer.
Like there:
Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
One main idea why I've started that tool: Saving own time to check domains.