After having read the hints of other forum members I did some testing after having received the ominous warning myself:
You have to find out what the annoying titles in your K3b portfolio are. Apparently the program does not like special characters in the file titles.
Often they are NOT DISPLAYED CORRECTLY in the listing of the files chosen, but look rather "harmless" - i.e. like "normal" ASCII chars. One hint is that there's an extra space between the line containing the "bad" chars and the next.
To sort them out you should NOT BURN your choice of files directly but use "simulate" instead, which will simply stop in its tracks when it doesn't like a title (while trying to burn the DVD/CD directly resulting in the error message compels you to reinsert the media each time).
After a successful simulation K3b will let you burn your disk; you should not only remove the mark from "simulate" but also set one to "verify" your work in the same panel
.
Happy & succesful burning !