Maybe you won't need to. I had forgotten an old case I had with the same symptoms. Here is how it was resolved
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In this case, Data Protector 8 was an upgrade from DP 6.21 (although this is also true if upgrading from DP 7.x). In their previos installation, their IDB was NOT in the default location
ProgramData -> Omniback -> db40
rather
Program Files -> Omniback -> db40
When the upgrade was made, it turned off Archive mode, causing this error
So, try this:
Stop Data Protector
Edit the file
ProgramData -> Omniback -> Server -> db80 -> pg -> postgresql.conf
and look for the line
archive_mode = off
Edit to line to read
archive_mode = on
Save the file, start Data Protector, try the IDB backup again. If it still fails, go ahead with your case, but, include in your case documentation that you tried setting 'archive_mode = on', and it didn't help