Hello.
Im using Duplicati for a while now, mainly as a Docker Container but also tried it on a few machines as a Windows or Linux Service.
I was searching for the same solution an the OP and the closest one was this thread.
After long searching, trying and configuring, i reinstalled my Mint VM bc i thought it could be a Cinnamon thing and tried the 2.1.0.110 Canary of Duplicati.
And: the tray icon shoes up with the startup parameters “–no-hosted-server --webservice-password=XXX” and it also opens the webui!
Only thing is that it always resets the webui color to light mode, but somehow this was my solution.
Probably this will be the fix in future stable release, but i wanted to take some words on that after i was frustrated months long.