So this problem started surfacing with a particular player a few days ago. They started crashing when opening their inventory. The fix, then, was to download the character file, remove a particular helmet, reupload it, and have a DM remake them.
The problem came back later though. It seems to be related to helmet with the appearance model 28.
This particular helmet lacks an icon. When crafted, it shows the default pot helmet icon.
After crafting multiple of them, dying them a bit, relogging a few times, changing areas, etc, I was able to consistently reproduce the crash. Without PoTM haks loaded, the crash didn't occur; I was able to load a character that would crash with our haks but not without them.
Possibly the helmet is simply missing its icon in the hak it's from. Or, the model itself might be corrupt, too. Either way, right now, do not use this helmet appearance.