AFAIK, this is known bugs and it has been written on the RELEASE NOTES:
One of the issues we're aware of is that k3b hasn't been working as well as the KDE3 version for some people (it works fine here, but we have heard some reports), and there are some other KDE applications where people still prefer the old versions.So if you need a K3B as a burning tool, install kde3 packages in /extra (which is located in disc 4) and install the old K3B version. It should work normally.
To help with this, there are KDE3 compatibility packages that can be found in /extra/kde3-compat/. With these installed, most KDE3 programs will run. A KDE3 version of k3b is also included in that directory.
There is also a good post about how to burn DVD iso. Some people failed to burn at 16x, so drop it to 4x or 8x and it should be working.