You have an interesting combination of a Ubuntu 16.04 LTS mono on a Ubuntu 18.04 LTS system. Although I’m not familiar with Ubuntu, I wonder if this system was 16.04 when you installed mono?
https://packages.ubuntu.com/xenial/cli-mono/
mono-complete (4.2.1.102+dfsg2-7ubuntu4) [ universe ]
conplete Mono runtime, development tools and all libraries
https://packages.ubuntu.com/bionic/cli-mono/
mono-complete (4.6.2.7+dfsg-1ubuntu1) [ universe ]
complete Mono runtime, development tools and all libraries
In both cases mono is not in the main repository, but in universe.
What are Ubuntu Repositories? How to enable or disable them?
I’m not sure where your mono is from, but Duplicati Prerequisites call for the latest mono for your OS at https://www.mono-project.com/download/stable/#download-lin and next Duplicati beta needs at least v5. There was at one time a bug in v6 that broke OneDrive, so that’s an extra issue if you happen to use it.
Among top results from Google search “MissingMethodException: Method ‘Array.Empty’ not found.” are:
ubuntu 16.04 mono Method ‘Array.Empty’ not found after nuget 4.5.1 #6790 (and mono 4.6.2 fixed this)
Summary: .NET 4.6 running on mono, Method ‘Array.Empty’ not found. (Win and Linux) has explanation
You then attempt to run this compiled application on an early build of .NET Framework or Mono, where Array.Empty does not exist (source notice that it says Available since 4.6)
I’m not sure why it used to work. Is it possible it started failing at your 2.0.4.23 upgrade? Possibly it was built differently even though the only difference in Duplicati source code from 2.0.4.5 is a new warning?
So – one option is to update mono as @birdypme suggested (thanks for it, and welcome to the forum).
Second option is to downgrade Duplicati, especially if you think the upgrade might have introduced this. Downgrading / reverting to a lower version shows the places where updates might be on Ubuntu Linux. Your base install may be in /usr/lib/duplicati. An install has a changelog.txt file. Version number is at top. Please see if you can identify your base install and whether you have any (can be several) updates to it. After you figure out what you have and where, we can talk more about the things you can do for testing.
Third option is least attractive, but let me ask if you have any export of your backup jobs in case needed.