Did your CDI ever change its whining tone?
 

Last Friday my SC suddenly died - no spark (fuel injection was ok).
The CDI was whining, but had a slightly higher tone than usual.
I pulled the distributor and measured the green wire and the magnetic
pickup, all fine. Then I connected the distributor without plugging it
into the engine, so I could turn the distributor gear by hand. I had
the ignition on and watched one spark plug. Every time I touched the
distributor, the tone of the CDI was changing from the unusual high
tone to the normal tone! So... does an SC distributor need a ground
connection? I cleaned and reinstalled the distributor and the locking
nut, now I measure a good ground connection to the engine case.
The car is running again.

It this normal? Can it be an indication for a faulty coil inside the
distributor? Or a faulty CDI?

Robert,

No, the only ground required is provided by the shield of the green coaxial cable.

Unless that cable has been changed fairly recently, I would change it now. The fact that it died, and that the CDI 'tone' was affected by you handling the distributor seems to indicate that the trigger signal from the pickup coil is being interrupted, and the most likely problem is the green cable where it goes into the distributor body. A replacement 'pigtail assembly' is available for about US$30. Alternatively, the entire cable to the CDI unit could be replaced with Teflon-sheathed coax from an electronic supply outlet.