Insert Statement Causing ORA-01858?

i'm executing a load test using the below statement:

INSERT INTO BPMBI.PPROCINSTANCE SELECT * FROM BPMBI.PPROCINSTANCE_BKP WHERE ROWNUM < 501;
COMMIT;
EXIT;

I've been using the same for a few days without any error while now wehn i try to use the same i get below error:

SQL> insert into pprocinstance select * from pprocinstance_bkp where rownum <= 500;
insert into pprocinstance select * from pprocinstance_bkp where rownum <= 500
*
ERROR at line 1:
ORA-01858: a non-numeric character was found where a numeric was expected.

I've been trying to check what the issue is and when i execute the rownum < 140 works fine while rownum < 150 is showing the same error.