VPN 800 Error: Connection failed.

I have set up VPN server at home. When I connect from one of the pc, works ok. When I try to connect to other domain’s VPN server (my friend’s), it doesn’t work. But when I connect from my server, it works. When I works from the server, why not from the home network PC. I didn’t understand.
I have disabled all the firewalls. I have forwarded ports for PPTP which is 1723 and 500 for L2TP/IPSec.
Can anyone help me please.