Hi Christophe,
You should be on version 1.1.1.06 to start.
Do you have the Port Address Translation rules entered in Setup>Port Address Translation?
As far as the error 51, check out this link and see if that helps.
http://www.firewall.cx/cisco-technical-knowledgebase/cisco-services-tech/1038-cisco-vpn-client-error-51.html
Regards,
Chris