Oscilloscope ground-probe causing test circuit to malfunction
I assumed it may be related to earth fault current, but both equipment supplies are properly earthed.


You cannot connect oscilloscope ground to anything that is not at mains earth potential.

You will get the following short circuit through your oscilloscope:







simulate this circuit &ndash; Schematic created using CircuitLab

I recommend this EEVblog episode:
EEVblog #279 - How NOT To Blow Up Your Oscilloscope!
(since you are lucky the diode failed first)



I must add that measuring on these live circuits is dangerous. You are supposed to use an active high voltage differential probe. A Tektronix P5200A for example.

There are also many other solutions to evade this problem, but since there are many safety caveats on these methods, I will not list them.