2nd session on Data DVD burn screws up DVD

I am using Nero OEM 6.6.0.18 with a Toshiba CD/DVDW SDR5372V drive on WinXP SE2.
I usually use it to create data backup DVDs. The last 4 times I have made these backups, something bad happened.

I make a fresh backup DVD, which works fine. I also tell Nero not to close the session, do that I can add to the bakup in the future. When, at a later date, I add data to the backup DVD (using the Nero Start Smart window and telling it to make a Data DVD), everything seems to be working OK, and at the end of the session, Nero tells me that the DVD has been burned succesfully, and I exit from the session.

However, when I try looking at the files on this DVD, Windows tells me that it can’t read the DVD or see any files. In other words, not only the data that I just added, but also the data from the previous session (which was OK before the added burn process, has been lost!

This is a recent phenomenon. Has anyone else experience it? Does anyone know what to do about it? Should I try different burning software?