Cause:Due to Oracle Flashback operations, Database is not able to dynamically tune the undo retention period effectively.
1)You can include CONSISTENT=Y parameter as in the case of Export(EXP).
2)Try to increase the value of UNDO_RETENTION parameter to match the length of the longest Oracle Flashback operation.
3)Schedule long running queries after your required operation.
4)Increase size of UNDO tablespace.