The problem is that the source address of those connections is always set to 10.0.2.1 instead of their actual IP (the gateway used in NAT mode, the VM's IP is 10.0.2.15).
I have encountered this problem in the past and I could solve it by shutting down the machine and running a command that went like this:
Code: Select all
VBoxManage --natalias or --nataliasmode <something>