Welcome to the forum @hermankopinga
This could use help from someone who knows the area better, but FWIW here are some more clues:
Change logs for libappindicator source package in Noble
Drop support for GTK+2.0 (libappindicator1, libappindicator-dev,
libappindicator0.1-cil, libappindicator0.1-cil-dev).
suggests that substitutions sometimes work. Next Duplicati Beta is possibly about to ship, but this issue sounds potentially serious, and Canary test couldn’t spot it easily because this Ubuntu came out April 25.
For a test, perhaps you can just unzip the .zip version then try a mono Duplicati.GUI.TrayIcon.exe