As a side issue, Linux Mint Releases shows their only releases are LTS releases, based on Ubuntu LTS. Other distros such as KDE neon take the same approach of building on a stable base, but stability leads sometimes into difficulty of change e.g. to a newer mono version either by distro or bypass-distro efforts.
For any who were hit by this problem, did the new mono come from your distro’s default repos, or did the https://www.mono-project.com/download/stable/#download-lin addition of mono project’s repo lead you?
This ties into a question of how current the mono on an LTS distro will be, in case Duplicati needs newer.
Survey for Linux users, which version of Mono do you have installed?