oper_20_4 is left over from an upgrade from 5.x to 6.x It should have been fixed back then.
There is a 'merge table', one of the tables in NBDB, which was to deal with 'probelm' entries between the upgrade to 6.x, where the entries relate to tapes.
File 731.dat in the nbcc output is the merge table in the version of NBU you have (.dat files can change between versions). You can also list media in the merge table with nbemmcmd -listmedia -mergetable - the outputs should match.
If I rightly remember, you have two conflicting entries for the same tape, only one will be correct.
Technically, nbcc is only needed to upgrade from 5.x -> 6.x, after that it is optional, that is, nbcc issues when upgrading from 6.x to higher shouldn't cause a failure /problem with the upgrade (despite there being an nbcc -upgrade option). Not fixing nbcc issues can however cause issues with the operation of NBU and can lead to dataloss.
Merge table entries are one of the things that are meant to be fixed, not really optonal - nbcc stops repoting any futher issues until oper_20_4 issues are resolved.
The only supported way to fix this is via nbcc.
Given there are merge table issues, I suspect there will be many other nbcc issues - some of these could put data at risk and so the only real way forward is to get a valid support contract and work through nbcc properly.