Dmitry, can you please look at this. Tari has mentioned this one before too. It sounds like we have situations where the sequence number field of VCenterReportunit is not being initialized. It should be set to the Reportunit KeyId when a new record is created.
Having said that, there isn't really a problem here. If you are not using sequence numbers, then the field is completely ignored by the system. We should probably hide the field in the list views in fact. If you are using sequence numbers, then you just need to regenerate them after you generate ballots (as usual).