2609806 - "The project is not in DTR, but there are readonly files!" when trying to modify a project in NWDS

After checking out a project from NWDI and trying to edit it, you are greeted with the message "Modification Not Possible - The operation requires the writeability of the following Objects:" and the error "The project is not in DTR, but there are readonly files!"

nwds, dtr, readonly files, modification not possible, writeability , KBA , BC-CTS-DTR , Design Time Repository , Problem