Assembly errors are not my area of expertise, but to gather info, what was your old Duplicati version?
C:\Program Files\Duplicati 2\changelog.txt will have that information at its top, because the update is written to (probably) C:\ProgramData\Duplicati\updates, where the old version finds it and launches it.
There were some early September Canary releases that had symptoms similar to your assembly one, and I don’t know if there is some way that a latent Program Files problem could relate to update issue.
Easy downgrade may or may be possible depending on what your previous version is. Fortunately the update looks like it might not have run far enough to upgrade the databases and block the downgrade. Seeing new backup files in your %LOCALAPPDATA%\Duplicati folder would mean it updated that DB.
Downgrading / reverting to a lower version talks about downgrading by removing new version’s folder, however another option would be to install Duplicati from the .msi file to see whether it starts properly.
Having installed 2.0.5.1 in Program Files, you could also remove it from the updates folder to be sure there’s nothing odd resulting from the two-step Duplicati launch design. Ideally an expert would arrive, and debug it better than I’m doing, but I don’t know how urgently you need to get backup going again.