DBMS_LOB.APPEND RAISES ORA-22275 WHEN CLOB RETURNED BY XMLSERIALIZE() GETS APPENDED TO ANOTHER CLOB
REDISCOVERY INFORMATION:

Error signaling Function: koklnflushint

The fix for 32008819 is first included in 19.11.0.0.DBRU:210420 (APR 2021) DB Release Update(DB RU)