Synaptic does not show Packets anymore

[BlackMac]

I have installed Ubuntu on my G4 Mac and everything ran fine in the
first place. I tried to get DVD Playback working by following the
instructions from the WIKI. After figuring out that one of those
repositories is only available for i386 I deleted it (the repository)
from the list. Now Synaptic does still find updates and does still
appear to "know" about all packets (apt-get works and Synaptic still
shows me how many packets it found), but it does not display them in
the list. I can't select anything anymore. When I try to select
another font for the application (inside Synaptics settings) it
crashes. Any ideas?

Thank you
Stefan