Problem with second session of data DVD

I am using Nero Burning ROM 6.6.1.4 with a Toshiba CD/DVDW SDR5372V drive in a WinXP SP2 system. Lately, it has exhibited a strange bug:

I use the DVD to back up my files, so that I write data to them. The first time I write to a disk, everything is fine, I can read the data without any problems. However, when I write to the DVD disk in a second session (I’ve kept it open for writing more sessions), the Nero software says everything is fine. But when I try to read from the DVD disk (using the same DVD drive), I get a pop-up window that says:

Disk is not formatted. Windows cannot read from this disk. The disk might be corrupted, or it could be using a format that is not compatible with windows.

And the disk, which is a backup disk, seems useless. But when I take the disk to another PC, and a different DVD drive, I can read the data just fine. I updated the Toshiba DVD Driver, but that didn’t seem to help.

Anyone know what the problem could be?