It probably ran out of retries when trying to get to Server on 8200 which was found to not be working.
Below points out that retries are finite when server isn’t there (or in your case, can’t be connected to).
It sounds like your Duplicati.GUI.TrayIcon moved from first-try 8200 to 8300 all by itself, which usually means something else got 8200 first, yet you find no Duplicati server on 8200 with browser or netstat.
Did you test the way I showed, so that any process having anything to do with port 8200 would show?
Ubuntu 18.04, Duplicati Service did not start anymore was fixed, possibly by restarting the system, but other things were done too. You can also see how you could edit in a custom –webservice-port to test, editing the service startup as above. For example, move it to 8400, restart, and see if you can connect.
Or if convenient enough, restart the whole system to see if some transient use of port 8200 goes away.
If the service doesn’t stay up at boot, try changing it from Automatic to Automatic (Delayed start).
Windows Service fails to start #2717 is my explanation, and has a couple of workarounds one can use.