HTTPS authorization using obsolete RSA

[jaras]

I got a site using letsencrypt and when checking my certificate I get a warning from Chrome saying the authentication is using an obsolete key. According to this discussion it should be a configuration of the webserver. So where can I find it? I use the built in domoticz webserver on linux ubuntu.

The connection to this site uses TLS 1.2 (a strong protocol), RSA (an obsolete key exchange), and AES_128_GCM (a strong cipher).

What I found so far, "this is about authentication, not about encryption" or most likely the key exchange part and "Preferably RSA should only be used for authentication and the key exchange should be performed using DHE or ECDHE."