I made further investigation and found that after export - import there were more records in the register QualConVc than before the export (!!!).
I exported a single register QualConVc via DB Maintenance module from the older version and compared it to the same register exported from the new version which I imported the Text backup into. I can see that after the export-import the last record was a duplicate of the previous record except the SerNr field. In the old version I had 75 records, in the new one - 76 records...