It's probably because the host cache used to bootstrap is now too old. You may want to Google for GWebCache (or BeaconCache), then pick a Gnutella host IP and try to connect to this host manually. I remember Gnutella2 connection was failing, so I fired up Shareaza, noted a G2 hub IP and manually connected to this hub with Gnucleus: Gnucleus then updates all its data and works like a charm after that, no need to repeat that again.

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Shareaza[dot]com was hijacked and now distributes an application pretending to be ShareazaV5. In fact, ShareazaV4 and V5 are relooked clones of BearShare and iMesh and connect to their centralized and filtered network. They are totally unrelated to the genuine, open source Shareaza which moved to http://shareaza.sourceforge.net/