Error PRJ0006: Error saving project to '[file]': [reason]
This is a generic catch-all error that occurs when a problem occurs while attempting to save a help file builder project. If you have been modifying the project file by hand, it could indicate that you have introduced an error into the help file properties.
Since this is a catch-all error, refer to the reason description for information about the cause of the error and how you might be able to correct it (i.e. permission issues, out of disk space, etc.).
If you have been modifying the help file project by hand rather than by using the help file builder standalone GUI, back out your changes and try to reload the project, make any necessary changes, and save the project. If you have not made any such modifications, open an issue in the Sandcastle Help File Builder project on GitHub containing the details from the error message. Attach the affected project as an example.