I found an old gaming pc and set it up. I installed Elten and now I always get an error which says that Elten couldn't fond the server-encryption-key and that this might be a server made by hackers. I can still connect to elten by pressing yes, but what is this? How can I disable that?

so basically, on the website, not all connections are being ported to https.
basically, some connections are going through http.
this does need to be fixed, and if pajpers is willing, he can easily block connections via port 80 (http) and only allow https connections. it's not that hard to do. the OVH box running elten also has the ability to block ports at the network level with OVH's firewall.
see this artical:
https://help.ovhcloud.com/csm/en-dedicated-servers-firewall-network?id=kb_article_view&sysparm_article=KB0043448

personally, I would only allow connections to ports 443 and whatever else you need to be exposed to the internet. for SSH, I would close that off to a VPN server via wireguard or use something like talescale.

IMO ssh does not need to be able to be exposed to the internet in anyway whatsoever.
he could also use cloudflare tunnel if web access to ssh and some other important stuff is needed.
I don't know about port21, as I think that's actually needed to do file transfers

But how can I get away this stupid warning when opening Elten?

One of the things to check is if the system time and date are correct. That cleared it up for me.

Yes, it shows the 2nd of December 2024.

Sorrry, i just checked and found out that time zone was set to amsterdam. I changed to vienna and now it works.

Yep, I thought that might be it.