Error while using ping and ssh : Cannot allocate memory

I hope this is the right forum.
I have a virtual ubuntu webserver running on my MacBook (Leopard) using VirtualBox. When I use the wireless router for my home network everything works fine. For the installation see: http://blog.johan-mares.be/ict/linux/installing-a-virtual-ubuntu-lamp-server-on- mac-os-x-using-virtualbox/
The moment I want to use the network my MacBook provides (192.168.2 subnet) by selecting the en0 Ethernet as host interface in VirtualBox so I can use it when I am offline it doesn't work anymore (whether I am offline or online via Airport): I cannot ping or ssh from my MacBook to my virtual server (whether is has a dhcp or static network config): I get
ping: sendto: Cannot allocate memory
or
ssh: connect to host 192.168.2.5 port 22: Cannot allocate memory
When I use dhcp config in my virtual ubuntu server, every time I restart my virtual the ip address is incremented by 1, but still I cannot connect to it.
I haven't got a clue where to look or what to look for.
Any advice would be welcome.