I can't find documentation to confirm this, but I suspect that under the covers the Interactive Report is loading the CLOB data into a PL/SQL VARCHAR2 variable, which will be limited to 32K bytes. As long as your CLOB data doesn't exceed that it's probably OK, but if bigger you will get the ORA-06502 error.
Rather than try to show the full CLOB contents in the report (which won't look good anyway for large amounts), you could show perhaps the first few hundred characters and have a link to display the rest in a modal page (for example). There you can read the CLOB data in chunks and use htp.p to write those chunks out in a PL/SQL region. Alternatively the link could download the CLOB data to a file.
