My web server is (include version): Apache/2.4.57 (Unix)
The operating system my web server runs on is (include version): Arch Linux
My hosting provider, if applicable, is: Contabo
I can login to a root shell on my machine (yes or no, or I don't know): yes
I'm using a control panel to manage my site (no, or provide the name and version of the control panel): no
The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot): certbot 2.6.0
After using sudo certbot renew all sites are insecure. I use this server for a long time and never i had a problem with renew the certificates. I try certbot certonly and generate new certificates. Restart apache and server failed.
https://www.projekt-hirnfrei[.]de works OK for me, has a valid certificate.
There is a separate issue that your site is listed as unsafe by Google Safe Browsing (Google Transparency Report), but this has nothing to do with your SSL certificate.
Please follow rg305 instructions but also quit getting more certs. You are almost rate limited if not already. I see 4 certs issued today for that name (you only get 5/week and sometimes the logs are late reporting them)