The best advice I have is for you to re-download and re-convert this card. When/if you do that, remember to UNCHECK the Don't Duplicate box in the Data Centre Tasks section to FORCE RDSS to re-download a fresh card file and FORCE a re-convert of that new file.
Using RDSS V2.1.049.11, I assembled all DMR races today at once: no failures. Then I went to each individual race and re-assembled it by right-clicking on the race # on the Race Summary screen: again no failures. I tested exporting a sample of a bunch of the races, sadly, with no failures.
Some unexplained effect during either download or convert of that card has resulted in data which is not responding normally to RDSS operations. I know the card is half over, but perhaps you could try that and see if it makes a difference.
I don't know if you have another computer, but it would be interesting to see if similar behaviour occurs in RDSS on another computer of yours. (It did not on 2 computers of mine).
If you can demonstrate repeated sessions where similar behaviour happens, either with Del Mar or with other race cards - first to yourself - then I would appreciate you contacting me by email and arranging a remote meeting for me to see what is happening on your computer. Aside from that, it is a mystery, frustrating, I accept, but something I have no other advice for at the moment
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Ted