@user4286 I haven't used the Home Assistant integration but do you know if it leverages a Telnet integration? If yes there is your problem. Denon and Marantz receivers only allow a single telnet connection at a time unlike other brands - My son's Onkyo allows 6 I believe for example. So if your HA is communicating with it via telnet that would prevent HE from connecting and vice versa.
My advice would choose one and then expose that device to the other if you really need to control it from both automation platforms.
To test this unplug your Denon receiver from the wall for 30 seconds and plug back in. Now go to your HE device and click initialize and it should now connect since the HA connection is now broken. Then you will likely get connection errors in HA.
