[Skimmer]

It's because the user is already registered. See this.

pawn Код:

else
{
    SendClientMessage(playerid, COLOR_WHITE, "Contul deja exista in baza de date.");
    KickEx(playerid);
}

I've also noticed a bug. In fact if you type 31 or more characters to register, the server will still call OnPlayerRegister function.

pawn Код:

if(strlen(inputtext) >= 31)
{
    SendClientMessage(playerid, COLOR_WHITE, "Password is too long (30 Chars max).");
    ShowPlayerDialog(playerid,37,DIALOG_STYLE_PASSWORD,"Server Account","Nu exista un alt cont cu acest nume.Creaza un cont folosind o parola mai jos\nVa rugam scrieti parola !","Register", "Next");
    return 1;
}

Change the return to

pawn Код:

return 0;