Whilst LW is CLOSED, delete the LW preferences folder but first move out the Downloads.dat file to a different location. Then open LW up again, close it again. Then replace the new downloads.dat file with the old one. This will ensure your old downloads history is retained. Deleting LW Preferences folder in OSX.
LW 5 has the ability to find fresh people to connect with much better than LW 4 did. BTW if you use your OSX firewall, remove the old LW listing & add the newer LW to it.
OSX Firewall settings for Leopard

Dear Lord of the Rings ! thank you but inspite of your fast answer, I had already thrown all data/preferences about LW 4, so without keeping any of the old data I could not follow your advice totally, and now I just redownloaded the version 5.5.8, allowing it in the firevault... BUT still it doesn't work, so if you have any patience left could you just tell me what to do now I guess as a new user of LW and not any more as an old one.