[Misiur]

In fact, use MAX_PLAYER_NAME + 1 for loginname.

Now, first of all, when you are using string without any parameters inside, then don't use format.

pawn Код:

if (gPlayerAccount[playerid] != 0) 
{ 
    new 
        loginstring[128],
        loginname[MAX_PLAYER_NAME + 1]; 

    GetPlayerName(playerid,loginname,sizeof(loginname)); 
    TextDrawShowForPlayer(playerid, TextdrawConnect2); 
    TextDrawShowForPlayer(playerid, TextdrawConnect3); 
    format(loginstring,sizeof(loginstring),"{FFFFFF}Salut, {2F991A}%s {FFFFFF}Ai deja un cont pe Comunitatea Union-Zone!",loginname); 
    SendClientMessage(playerid, COLOR_WHITE, loginstring); 
    ShowPlayerDialog(playerid,12346,DIALOG_STYLE_PASSWORD,"Login","Acest cont este inregistrat.Te poti loga!","Login","Exit"); 
}

And the problem is somewhere else, not here.