Fixed by adding a conditional to all output copy steps, so no additional build errors are generated when an output file is missing, or no XML docs are generated.
Triggered a binary release for the editor.
Note that this only fixes the issue for projects that start off fresh, since the editor won't touch existing source folders / solutions. They'll have to be fixed manually.
Summary
If no documentation file is generated the build will fail because its trying to copy a file that does not exists.
How to reproduce
Create a new duality project and disable generating a documentation file and build the project
Analysis
Condition="Exists('$(OutputPath)$(AssemblyName).xml')"
to the copy task that copies the documentation file