SQL & PL/SQL

Hello out there,
I have a problem calling a stored procedure via dbms_scheduler that pulls some rows over a public database link.
The setup is the following:

create public database link mediabase using 'mediabase';

create or replace procedure hole_kurse as
mdatum date;
begin
   select max(datum) into mdatum
   from dt_wechselkurs;

   insert into dt_wechselkurs l
      (waehrung,
       datum,
       wechselkurs)
   (select
       r.waehrung,
       r.datum,
       r.wechselkurs
    from
       dt_wechselkurs@mediabase r
    where
       datum>mdatum);

    commit;
end hole_kurse;
/

begin
   dbms_scheduler.create_job(job_name          =>  'wechselkurse',
                             job_type          =>  'STORED_PROCEDURE',
                             job_action        =>  'hole_kurse',
                             start_date        =>  sysdate,
                             repeat_interval   =>  'FREQ=DAILY; BYHOUR=7; BYMINUTE=0; BYSECOND=0');
   dbms_scheduler.enable(name => 'wechselkurse');

   commit;
end;
/

I can access the database link in SQL and I can call the procedure hole_kurse from SQL without any errors. But the job fails writing "ORA-01017: invalid username/password; logon denied" into alert.log. I also tried using dbms_job which used to work with Oracle 10g but now fails with the same error.
My Oracle version is 11.2.0.2 64bit on Window Server 2008R2.

So what do I have to change that my job will run?

Many thanks in advance,
dhalek