Unexpected response code 555 with Volley Request in Android Studio

Question

I'm tying to call a web service that I created, but I'm carrying on to obtain the http error 555. I did the web service using ords (oracle) to develop it. I did many others of those and I didn't find any problem like this. This is the web service wrote with Oracle sql developer and GET as Handler:

 begin
declare
p1 varchar2(4000);
p2 varchar2(4000);
p3 varchar2(4000);
p4 varchar2(4000);
p5 varchar2(4000);
p6 varchar2(4000);
p7 varchar2(4000);
p8 varchar2(4000);
p9 varchar2(4000);
pdt date;


outrc number;
outms varchar2(4000);


outs1 varchar2(4000);
outs2 varchar2(4000);
outs3 varchar2(4000);

outn1 number;
outn2 number;
outn3 number;
outn4 number;
outn5 number;
outn6 number;

begin
     input parameters
    p1 := :p_p1;
    p2 := :p_p2;
    p3 := :p_p3;
    p4 := :p_p4;
    p5 := :p_p5;
    p6 := :p_p6;
    p7 := :p_p7;
    p8 := :p_p8;
    p9 := :p_p9;
    pdt := :p_pdt;

    outrc := 0;
    outms := ' ';

    CASE :p_cmd 
    
        WHEN 14 THEN
            BEGIN         
                apex_303.pro_conferma_dati(p1,to_number(p2),to_number(p3),REPLACE(REPLACE(p4,'%3D','='),'%3B',';'),outrc,outms);
            END;

    END CASE;

    :p_rc := outrc;
    :p_msg := outms;


    end;
end;

This is my query parameters :

p_cmd=14&p_p1=4&p_p2=1000048819&p_p3=03394100140105&p_p4=BAN_NUM=1;BAN_COD=null;SCARTI=0;

That query parameters in Postman works fine without changing code of my web service. I can call this:

apex_303.pro_conferma_dati(p1,to_number(p2),to_number(p3),p4,outrc,outms);

and I get back status code 200. I know that the problem is here: p_p4=BAN_NUM=1;BAN_COD=null;SCARTI=0;. There are many semicolons and uguals, but I can't undertstand what it's the right way to encode. Can anyone tell me the right way to manage the query parameters?