Installed Duplicati followed by Mono, and did not work.
Tried the other way around too (Removed Duplicati, and installed it again - where Mono was present and running on the system). Restarted the system to give it another chance, but same results.
Details:
DS414 with DSM 6.2.1-23824 Update 4
Mono 4.6.2-0096 (Beta-version from the official Synology-store)
duplicati-2.0.4.5_beta_2018-11-28.spk