When building source-controlled projects that have conceptual topics, DocProject may copy read-only files into the buildhelp\assembler\DdueXml working folder. The next build will fail while preparing the target folders since DdueXml cannot be deleted while it contains read-only files.
Original discussion and temporary workaround:
http://www.codeplex.com/DocProject/Thread/View.aspx?ThreadId=29857