Dear support team,
My Mac runs on 10.14.6.
Duplicati does not start!
Do you have an older version that works on this system?
Best regards
Roland
Same here - just started happening today.
I may have updated mono (via brew) and broke something, but I don’t think so this time.
mono: /usr/local/Cellar/mono/6.12.0.122
Duplicati: 2.0.6.3
mono --version
Mono JIT compiler version 6.12.0.140 (2020-02/51d876a041e Thu Apr 29 10:44:55 EDT 2021)
but… monop --runtime-version
runtime version: 4.0.30319.42000
MacOs 10.15.7
Happy to provide more information, but I don’t know where to look… 
– Mike
Golly. As soon as I post this (I had rebooted once, but to no effect), and turn my attention to something else - and related or not, I started my TimeMachine backup - duplicati finally showed up on my task bar.
Seems to be working, but I’ll be giving it some side-eye for a while…