No, not really. After I looking around a little bit, making some experiments with missing TXT-Records, I found the solution.
All CSS, JS, images and so on was loaded with 'https://www.optiliga.de/{the file}'. However, the name of the certificate is correct 'optiliga.de'. After I change the...
10 days ago I installed a new Server and move my one of my Domains https://www.optiliga.de to this installation. I saved this via plesk with a 'Let's Encrypt' certificate.
All works fine.
Yesterday I looked to the domain and see, it is broken. In console log I find the message "failed to load...