Since I have installed the Mandriva Spring (2010.1), keeping my old /home partition, my Ktorrent doesn't work anymore. I have the message 'Unable to contact 127.0.0.1', and it can't can seed or leech anymore.

I ran it successfully with an another account. The problem comes just with mine. But my Ktorrent configuration seems to be the standard one.

When I turn off completly the firewall or I delete ~/.kde4/share/apps/ktorrent, nothing different happen.

When I was using Mandriva 2010.0, all was working quite good, and I didn't change any configuration in the software.

How could I please correct this ?

Try deleting the whole .kde folder. I had a lot of breakages when upgrading from 4.4 to 4.5 and that fixed them all.