If this would be a general weakness of the ADI-2 DAC, then you would find more users here with the same issue, but this is not the case.

I remember that with Firewire 400 there was a danger of damaging the circuits when plugging in or out if the plug got twisted or something like that. You may experience a similar effect from a defective cable or USB port on your laptop.
I can't imagine that the ADI-2 DAC should be the problem, especially if it was freshly repaired. I'd rather guess a bad USB cable or a bad USB port on your laptop. At least I wouldn't rule that out.

Otherwise I fully agree with you, that plugging and unplugging USB devices should not cause any issues.

While mostly known from FireWire, static electricity and very different ground levels between two USB devices can also damage the USB receiver chip. Every USB unit (should) has a protective circuit to prevent that - these are standard components build for exactly that application.

In case a dangerous voltage occurs the protective component can short that voltage and thus protect the USB receiver. It now depends on the amount of voltage and the power (time) of higher voltage if a) the dangerous voltage is just collapsing and that's it, b) the protective component is destroyed, c) protective component and USB receiver are destroyed.

In your case it seems that you have a very high floating voltage on your notebook, which is shorted to ground over the ADI and the units connected to its analog outputs. You should make sure the laptop is properly grounded, same with your monitors/amps. And maybe get someone with a voltage meter to check the current situation.

It also seems when you don't change the setup the next ADI will die soon...

So there is the reason it broke two times. The ADI's power supply is ungrounded/floating, thus no problem plugging it into your ungrounded notebook...

I think that's what Matthias suggests. But nothing in the manual implies that you shouldn't use a grounded power supply. In fact, it states on page 10: "the choice of power supply is not critical." Moreover, with the floating standard SMPS the ADI-2 is still grounded via the connected power amp and the voltage difference between the floating USB ground of the laptop and the ADI-2 USB remains a whopping 59V. So I don't see how my linear power supply is the problem..

I can't proove that. It is just a thought. Yours broke two times in that setup, thousands of others never have a problem. As I said the real cause of the issue is unclear. Thinking that ground here and there must be the same might be right for audio, but not for static sparks and such stuff.

The rest of what you said is correct and it is good to see a user who  knows what he is doing, and optimizes his setup on a factual base instead of hearsay and believe.

I may have found the culprit in my setup. My mains isolation transformer is a balanced toroid type that doesn't like mains DC-offset. So I installed a (Rod Elliott) DC-blocker on the (single phase) AC input of the transformer. But the mains ground must still be connected to the secondary center tap of the transformer, which is the 'new' ground for the powered equipment. In my home/system this introduced a DC voltage which audibly messed with the balanced zero or null. So I installed a capacitor (18μF 400V MKP) in the mains ground connection to the balanced center tap. This completely solved the audible DC problem but it introduced another; the capacitor may get charged enough to kill a ADI-2 USB chip.

Without this series capacitor, the mains ground connection introduces at least 5mV DC-offset/skew into the balanced output. To remedy any future static discharges into USB receivers, I bridged the capacitor with a 25KOhm resistor. Now the DC component of the balanced ground is around 0.01mV and I think it sounds even better.

The question remains why the ADI-2 died and my other DACs survived in the same setup. I did use the Intona isolator most of the time. When I blew the ADI-2 USB I connected it directly to an ungrounded laptop..

Correct. With the Intona in the USB chain this could not have happened because it galvanically/electrically isolates USB ground and signal connections. I use the non-industrial version (up to 1000V protection) which is cheaper.