Pretty easy to recreate:
CREATE TABLE exp_clob_test (ID INT, c_data CLOB DEFAULT empty_clob() );
INSERT INTO exp_clob_test VALUES (1, 'test');
COMMIT;
They try to export using the PL/SQL Developers tab. I get a small error box that says "write stream error". Perhaps its my environment but not sure where to look. Note: the Oracle tab export works fine.