Thank you for the answer. You are right, this seems to be affecting Windows users.
I opened the disc index.plist file on the disc which is an XML file that lists all the files and the discs involved in the backup job. I can confirm that while almost all slash characters are replaced with underscore, volume names for the restore were left untouched. This seems to be a small bug in Toast (but again, I still don't know which version) where it doesn't replace all the slashes in the config file before writing. If I am desperate enough I could probably create those DVDs with the new config file, but at this moment I will just wait for the photographer to resend me those 3 files.
This is the part from index.plist file, in case this is a new version of Toast, and developers are interested in bug reports:
<string>Disc 5_20_12 - Part 1 of 4</string>
<string>Disc 5_20_12 - Part 2 of 4</string>
<string>Disc 5_20_12 - Part 3 of 4</string>
<string>Disc 5_20_12 - Part 4 of 4</string>
Notice PC Labels are corrected but not PC Volumes.
They have separate entries for Macs so this disc would probably work for Macs.