这是成功解决问题的一个解答，网上考来的，自己看看吧！
What Oracle version has your catalog DB and the backed up DB (further called original DB).
Had the same error when i used a Oracle 8.1.6 for catalog DB and Oracle 8.1.6.2 as original DB. Seems like the exp command from 8.1.6.2 doesn't work correctly with 8.1.6 DB.
A solution would be to copy exp from catalog DB to bin directory of original DB as exp816. Then create a file .profile in the /etc/opt/omni/oracle8/<your ORA_SID>
directory on the server where your original DB runs. The file should include the following line:
ORA_EXP=<your ORACLE_HOME>/bin/exp816
Omniback then uses exp816 for the export of the catalog DB.