Error ORA-01555 by export space

2011-01-31 09:05:07,747 ERROR [http-10181-Processor37] [sf.hibernate.util.JDBCExceptionReporter] logExceptions ORA-01555: snapshot too old: rollback segment number with name "" too small
ORA-22924: snapshot too old

Confluence writes this error to the log file by export an space. Change the UNDO_RETENTION in Oracle DB to a upper value did not solve the error.