Macrovision is an artificial video error injected into tape signals that prevents copying. Since many normal video errors exist on old home recordings, sometimes over-sensitive macrovision detection on a DVD recorder will make the unit think the tape is protected, and it will stop recording.
You should search to see if there is a firmware hack to disable macrovision detection for that model. Video stabilizer boxes should help, too. Alternatively swap the recorder for a different model if you can.