It's about the weapon selection in game.
I have many weapons, in ONE character, and maybe this is the reason of "equipment disappear". For example, I have 8 one-hand weapons and one of them is Broad short sword. When I try to select it in game, I can't find it on weapon list.
I switch out of the game and check my equipment on website, and everything is fine.
Next, I set Broad short sword as one of my default equipment in character->inventory, rejoin the game.
Broad short sword shows up as I belief, but my two-hand sword disappear...
Maybe warband has weapon amount's limit?