This has been previously mentioned (Custom Fleet "Crash") but not with usable information so I hope to remedy that.
Symptom: Application enters a non-responsive state while said application continues to consume increasing amounts of memory leading to an inevitable crash of the entire system as memory reaches capacity requiring a hard reboot. (As very vocally mentioned by everyone who has made a post concerning this)
Causation: From what I can identify it strictly has to do with making a custom fleet, the Fleet Creator, where upon opening whether with the intention of creating a new fleet or merely inspecting the loadout of a stock fleet will risk this hang. It is not an immediate effect but has a delay of ~1-2 minutes between opening the creator and the symptom occurring.
As this is not an actual "crash" there are unfortunately no crash logs available.
It is not reproducible with %100 certainty, but pretty likely as there have been a few times I've been able to get in and out of the creator without the hang occurring (<20 seconds spent). Maybe duration spent in the creator affects likelihood of the hang?