Hi ObiOne,
I don't know if you solved this problem already or not. You stated that other vendors on the same profile can be posted correctly. Have you tried creating a new journal for this vendor?
If you have still the posted journals from other vendors, you can compare the LedgerDimension and DefaultDimension fields on the LedgerJournalTrans table. Is there any differerence in the reference ID (large integer value) between the lines that were posted for a similar vendor and the current blocking line?