Answer
Check whether the tablespace actually backing that CLOB storage, commonly a dedicated LOB tablespace, has run out of allocated space. Query dba_data_files (filtering to the relevant tablespace) to check each of its data files' current size, maximum size, and whether autoextend is actually enabled. If the tablespace's data file(s) have hit their maximum size or aren't set to autoextend, add a new data file to that tablespace, for example ALTER TABLESPACE <tablespace_name> ADD DATAFILE '<path>' SIZE 50M AUTOEXTEND ON NEXT 20M MAXSIZE <limit>;, to free up room for new CLOB content to actually be written. This kind of storage-related change should be done by, or with, a DBA, and during a maintenance window.