The Avid error “Exception: DISK_FILE_ALREADY_EXISTS, filename: *” is usually indicative of a corrupt or locked file either in the client machine folder located on in the project.
As long as the client machine folder is empty (or at least does not contain any Avid MediaFiles folders), it is safe to delete the client machine folder from the project.
After the client machine folder has been removed, reboot Avid and your project should open normally, creating a new client machine folder in the project.