ok i explain,

login with 2.6.7b3 :
login is fine, the position "BEFORE" u login is correct and "SAME" as u log out before
than u change to 2.6.7b4 :
FIRST join position is fine, and u can play and log out, but
after second login (its the firts time player ask for login location !) the player is set DEEPER as he was before he logout !
when player finally /login xpasswordx , he is ported so the CORRECT position before he log out.

So this is why Player DIE IN VOID cause they join the server at wrong location
and "sometimes" not always, the fall and die,
not possible to get items back.

AND ! when a player teleports to the player who died (he is safe in his house^)
the player who will teleport port to "OLD" position, and also DIE IN VOID

i think this happens cause u change this :

 Changes 2.6.7b4:
    Minecraft 1.3.1 compatible
    revert AsyncChat changes with old API ( temp fix)
    " add new pack coords method " <--------------------------
    better check of comand execution while not logged in