Hi @Frank_Nickel,
Thanks for confirming that this behavior exists outside of Roon as well. You might be right here, this type of behavior could also be caused by interference from the 2.4Ghz WiFi spectrum, since as you mentioned both Bluetooth and WiFi operate at the same frequency.
I have not seen this type of behavior often but it is possible. The other possibility here is that there is something going on with the bluetooth module inside of the Denon Envia itself. Since we confirmed that all of your devices stream as expected to the Roon Remote speakers output and issues just arise when bluetooth is added in the mix, I would try to see if Denon can possibly repair or replace the speaker for you.
I have also been searching other forums for similar issues with Denon Envia and have found some reports that the battery for the Envia can cause similar behavior. There are a few Amazon Reviews which also note similar battery behavior. Just to try and rule this out, I would connect the speaker directly to an outlet and have it charging just to verify if the same issue still occurs.
Thanks,
Noris