[Konstantinos]

pawn Код:

format(Query, sizeof(Query), "SELECT 'password' FROM 'users' WHERE 'username' = '%s' LIMIT 0, 1", DB_Escape(GetName(playerid)));
    Result = db_query(Database, Query);
    if(db_num_rows(Result))
    {
        db_get_field_assoc(Result, "ip", User[playerid][IP], 16);
        db_get_field_assoc(Result, "password", User[playerid][Password], 129);
        if((!strcmp(iP[playerid], User[playerid][IP], true)))
        {
            LoginPlayer(playerid);
        }
        else
        {
            ShowPlayerDialog(playerid, DIALOG_LOGIN, DIALOG_STYLE_PASSWORD,
            "xPS : Login", ""white"Welcome back to "#SERVER_NAME"\nYou cannot play the game if you don't login, Here's some several reasons:\n"red"• "white"Your stats will not save\n"red"• "white"To avoid ban evade!\n\n"red"NOTE: "white"If this is not your account, Please left the server and use another username!\n\nPlease input your account's password!", "Login", "");
        }
    }

Selecting only password and then trying to get data (ip and password fields). That's an incorrect way.

Also make sure that the query's lenght is enough in OnPlayerDisconnect.