Hi,
just to let you know you're not alone, I also got a new Echo Dot this week and have exactly the same problem.
Did you by any chance set up the Sonos skill on your app/device? I set it up and removed it and the Sonos devices but the problem is still there just with the Echo Dot device, it also occured on the Sonos devices when they were set up. I don't know whether the issue was there before I set up the Sonos skill.