Limited NPC functionality

[Yen]

Yeah, I'm pretty sure we had this problem at Providence, too.

Some players on Zodiac are unable to interact properly with NPCs.
They can call the attack function, carry out all the commands in it, but the triggeraction doesn't happen.
When they take damage, the 'damage' parameter is always 0.
In their inventory, they can see how many of each item they have.
However, the GUI thinks they have 0, and both the inventory and GUI use the same function to retrieve the quantities.

This is happening to maybe 10% of the players, and there's nothing they have similar, as far as I can tell.
They're all on Windows, two are on different versions, they're gold, they're on v4..

Has anyone experienced/fixed this?
Stefan help would be nice.