Hi,
If you can share the callstack, that would help us to know which line of code is trying to access the invalid data, and based on that we might be able to provide some suggestions about what scenario might be causing it.
You are probably getting the crash in different threads each time, because the system crashing is the one working with the physics thread state of the simulation. In Chaos, we have two states of the simulation. One is the Game thread state (this is the one you have access trough the existing API to interact with physics from Actor/Components), and we also have the physics thread state. That last one is the one used to advance the simulation using multiple threads, not just one (and not the game thread).
Proxies are part of the system used to sync back and forth between the two. I recently explained a bit how this works in another post Is Async Physics Tick really async? - #2 by sergio.gardeazabal .
That said,
If the code trying to access the (now invalid) proxy is doing it trough a particle handle object, you might be able to inspect in the debugger the DebugName property (if you are able to reproduce the crash with the debugger connected).
In Debug/Development builds all Chaos particles (Physics Bodies) should have a debug name property. Which is a string composed of the ActorName+PrimitiveComponentName+BoneID.