I’m getting these same errors also, Ubuntu 19.10 64bit intel with 2.0.5.1_beta_2020-01-18.
I just installed mono-complete (must have had -devel before) and will see if that fixes it. Only every 2-4 backups contains the warning, which is pretty bizarre.
2020-02-11 02:00:00 -05 - [Warning-Duplicati.Library.DynamicLoader.DynamicLoader`1-SoftError]: Failed to load process type System.Reactive.Concurrency.DispatcherScheduler assembly /usr/share/Duplicati/updates/2.0.5.1/System.Reactive.dll, error message: Could not load type of field 'System.Reactive.Concurrency.DispatcherScheduler:<Dispatcher>k__BackingField' (0) due to: Could not load file or assembly 'WindowsBase, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies.