Despite of my limited knowledge of C (My profession is Java programmer) I think I found the cause and the location of the PartyGammon bug. It is at the lines 3523 and 3524 (those were the line numbers in the snapshot of 6-9-2007) of the file
import.c, in the procedure ConvertPartyGammonFileToMat. The new matchscores are calculated here by adding pGame.s1 and pGame.s2 to the old scores. However, pGame.s1 and s2 contain absolute matchscores, not the points won.
This explains why the first game of the match is always imported correctly, the old scores were still zero so the absolute score is also the difference. So the lines 3523 and 3524 should be: s1 = pGame->s1;
s2 = pGame->s2;
I changed this at my own pc and now the import works.