For starters, Full is insecure. You should use Full Strict.
Then, I’d recommend to pause Cloudflare (bottom right), check if the site loads fine on HTTPS and talk to the host, if it does not. Once it loads fine on HTTPS, you can unpause Cloudflare.
In that case, yes Cloudflare does serve as Let’s Encrypt certificate but that’s perfectly all right, as Cloudflare uses a number of different certificate authorities.
If you are using Full Strict, you actually have a proper setup.
Dear sandro I wish you were right but I’m afraid that this ssl works on old windows 7 and even old android devices, let me check this and I get back to you …
And the certificate has expired. Just not yours, but the root certificate which gets updated with the operating system. Windows 7 not being supported any more, is now unfortunately left in the dark. You can manually update it or switch the CA.