Tonight I removed the wireless module from the Cosmos and installed it to my original Vive again: Played Boneworks for 4 hours, completing the "Streets" level. Game ran stable, no crashes. Steam VR two times lost connection to the HMD, but after restarting the HMD I was able to continue playing (the game itself had not been influenced by this issuses).
So, for me it's clear: The Cosmos is the Problem.
I found this post on Steam in the Boneworks forum:
Being flung into the void is a typical result of your controllers reporting NaN as their position - I get the same by launching the game with my controllers off, and then turning them on.In short, it's the physical hands suddenly interacting with the environment in ways that it shouldn't due to poor tracking.