The problem may lie in selected null values ??in combination with a CLOB-type column.
select valueVarchar c1 ,
valueClob c2 ,
valueVarchar c3 ,
valueVvarchar c4
of Table_1
union
select valueVarchar c1 ,
valueClob c2 ,
valueVarchar c3 ,
null c4
of table_2
I reworked the cursor. The first cursor is composed of four non-null columns. The second cursor selects three non-null columns. The null values ??were injected into the cursorForLoop .