[CentOS] Upgrade 4.4 to 5.0 (Error in transaction)

Hi!

I'm having a problem upgrading an existing 4.4 installation to 5.0. 

- The machine is a x86_64 architecture
- I'm upgrading from a DVD
  - I've used the same DVD to sucessfully upgrade another machine
    (therefor I'm pretty sure that it is OK)
  - The error occurs on both DVD-drives on the machine (therefor I
    think its not a problem with a particular DVD-drive)

I'm clicking through the upgrade dialogs. Then the machine prepares to
install the packages. At the end of this a dialog-box appears:

Error running transaction
There was an error running your transaction for the following
reason(s):

Apart from that text there is only a Reboot-button (no explanation)

On the other virtual terminals there is nothing unusual (last line in
Ctrl+Alt+F3 is "Preparing to install packages")

The /root/upgrade.log is empty.

My guess is that there might be some packages that offend the
installer (there are some non-standard packages on the machine, don't
shout at me: I know that's bad (amongst them a 32-bit firefox)). Is
there a way to find out which packages are the bad ones?

Any tips would be most welcome. Thanks

PS: What I'm trying right now is removing the worst offenders and retry
the upgrade procedure in text mode.