How to troubleshoot/debug Oracle permission problems
Another option (short of full auditing) is a servererror trigger for the schema in question (or the whole database).

drop table error_log;
create table error_log (error_time timestamp, username varchar(50), msg varchar(4000), stmt varchar(4000));

CREATE OR REPLACE TRIGGER servererror_trigger 
  AFTER SERVERERROR
  ON SCHEMA
declare
   sql_text ora_name_list_t;
   msg_     varchar2(2000) := null;
   stmt_    varchar2(2000) := null;
begin

  for depth in 1 .. ora_server_error_depth loop
    msg_ := msg_ || ora_server_error_msg(depth);
  end loop;

  for i in 1 .. ora_sql_txt(sql_text) loop
     stmt_ := stmt_ || sql_text(i);
  end loop;

  insert into error_log
  (error_time, username, msg, stmt)
  values 
  (current_timestamp, ora_login_user, msg_, stmt_);
end;
/


If it's for a single schema/user the trigger needs to be created under that user. You also can trap errors for the whole database by specifying ON DATABASE. In that case you'll need to create that trigger and the table as DBA I assume (haven't tried that).

Now nearly all errors that happen will be recorded by the trigger (see the manual for a list of errors that will not be reported: http://docs.oracle.com/cd/E11882_01/appdev.112/e25519/create_trigger.htm#LNPLS1992)