Just some additional information:
I'm trying to access a CLOB field in my database using the classes from Oracle.
Any other data (Strings, numerical values) are read just fine, it's only when I try to use the Oracle classes to access the CLOB that I run into an error.
Because I ran into some problems concerning transactions, I set up a no-tx-datasource that is used exclusively by my BMPs. Changing this to the other data source that is used by the CMP beans makes no difference, however.