Look for typing errors in the repository entries, especially the slash at the end of the URL or (my favourite) actually typing "N/A" instead of leaving the field blank (don't ask).

This "tool" was designed by a braindead *****: Would it have been that hard to code it so that it would skip incorrect entries, rather than grind to an oh so userfriendy halt?

Sheesh Nokia, simply porting Synaptic was too difficult?