You need to use the --no-hosted-server option when you run TrayIcon. Otherwise it will start another instance of the Duplicati server instead of connecting to the service running via systemctl.
With two servers running there may be some confusion. You can view the configured jobs in each server by going to:
http://localhost:8200 (this is probably the systemctl one)
http://localhost:8300 (this is probably the TrayIcon one)
Here’s a recent thread where someone discusses it how to run TrayIcon but have it connect to the systemctl service: