CPU 1 Vcore voltage error

I've gotten the same issues on the same setup (G5 Xserve running 10.3.9). I've been able to isolate this to heat and ventilation. Once the machine was moved to a better cooled and ventilated environment, the errors went away. And Recall that Retrospect is not the nicest piece of software. When it runs it will consume near 100% cpu which will increase internal heat. That makes the fans rev up and the whole unit heat up.

If you have the unit in a controlled environment, try lowering the temp a few degrees. If you don't get a small fan and either blow cool air at the front of the unit or make sure you are venting the hot air. As summer draws on, you will likely see the problem more frequently.

Hope this helps.