A single-beep GFI test failure is most often a transient failure. Tracking down the actual failure has been elusive, but may be related to low line voltage (208v instead of 240v) or an issue with the -2.5v (negative 2.5v) regulator in the GFI circuit.

Most often this will clear itself if the temperature or ... or the planet alignment changes. Simply unplug the box for 2 seconds, plug it in again, and it will re-test.

More rarely, the 0.1uF (105) yellow tantalum capacitor attached to the large -2.5v regulator on the board needs to be replaced with a 1uF (106) capacitor. Observing the voltage on a scope, you may find a sawtooth voltage output there. That's the telltale sign of needing a 1uF capacitor instead of 0.1uF.

You may find the GFI test resistors have cracked - put some solder on the edge of each one and see if they break in half when the solder is melted. Simply replace with a matching value resistor.