Sinamics S120 SLM 5kW Booksize Malfunction

Dear all,

We are in a serious situation when 2 SLMs, 6SL3130-6AE15-0AB1, have already been damaged and we are still looking for the cause. Hopefully, we can get some help from you to understand the phenomenon, before installing a new SLM and risking to damage it, too. The system consists of the SLM in question, along with the associated line filter and line reactor, 1 DMM and 1 SMM, both booksize compact and a CU320-2 PN that controls 3 axes.
The first SLM has failed at the moment when the client turned off the machine from the main switch (not the first switch off, but after a few ones). Smoke came out of it. On the next switch on, the "RDY" LED was lit orange and the "DC LINK" LED red. DC link voltage measured was around 180V. We have replaced the defective equipment and had our client install an early break auxiliary contact on the main switch to disable the pulses of the rectifier unit when switching off (initially they didn't have one). Unfortunately, the same happened to the new SLM as well, but now the RDY" LED goes red and the "DC LINK" LED orange. However, we couldn't reset this error as much as we tried, neither with X22:3 nor with a Power On.
What is happening?
Some relevant information to share with you:

1. Machine was tested at the OEM's place for about a month before shipping it to the final client. No problems occurred during this time.

2. At the client's place, the machine is supplied via an RCCB,https://www.schrack.com/shop/residual-current-circuit-breaker-40a-4-pole-30ma-type-ac-g-bc024103.html, that trips when the power is being switched off from the main switch. I already told them that this device is not suitable, but I still cannot explain the connection between the it's tripping and the breakdown of the SLMs. It may also happen that there is a power supply blackout, when everything has pulses enabled (motor modules also), and the SLM shouldn't fail, in my opinion. I'd like to emphasize that the first SLM has failed after several switch offs. Can it be a wiring  or commissioning problem, or rather the power supply or inadequate breaker? I tend to believe that it's the power supply along with the breaker, but I cannot be sure (and what exactly is not Ok with it?).

3. Unfortunately, we do not know what has failed in the first unit, because it was sent to service in warranty, and instead of getting a report of the diagnostics we received a new one. These information would have helped us, a lot, in understanding the phenomenon.

Best regards,
Simon Levente 

What is present in the fault history of the drive after a failure?  Does it show an E-Stop condition was met before low incoming power?

Does the system have a DC braking resistor, or network of them, to absorb motor energy?

Is the DC bus left up for as long as possible, or does it fault right away and try to dump all the DC bus plus motor energy into the line module's output state?

What section of the SLM's keep failing?  Input or output?