Oh well. I guess it gets harder. I’m not sure how far you want to go, but I’ll suggest some things.
“Duplicate upgrade to 2.0.3.3_beta_2018-04-02 fails to start”
is a somewhat similar situation that didn’t get solved, so possibly helping you will also help that.
It sounds like you manually start the tray icon. If you installed from .msi, did you tell it to NOT start up?
There are two spots that might cause starts, one on the feature selection, and one on the final screen
https://duplicati.readthedocs.io/en/latest/02-installation/
Are you doing basic things, or more advanced like command line, Windows service, environment, etc?
Actually, it might help to learn more about uses and history. Any known changes since this last worked?
When you watch Task Manager when starting Tray Icon, do you sort by Name? Anything else Duplicati?
What appdata folder are you clearing? I think it will vary. Mine is \Local\Duplicati (not the Roaming one).
In Duplicati appdata (or maybe elsewhere?) should be a control_dir_v2 folder. Does it have a file in it?
My system with Duplicati up has the file lock_v2, and it’s locked. Dragging to an open Notepad told me
“The process cannot access the file because it is being used by another process.”
Closing my Tray Icon made that lock file go away. You can check to see if somehow you got a leftover.
Actually, depending on how quickly File Explorer works, I wonder if you can spot the file briefly appear.
To debug even deeper, you could use Sysinternals Process Monitor with some filters set to limit output.
For example, you could monitor all Duplicati processes, or keep an eye on activity done to file lock_v2.
Unless Duplicati is totally imagining things, there’s something it perceived that is upsetting it. But what?
Good luck with your search.