Hello and welcome to my latest problem: VPN Routing doesn't work.
I've got a Server and two clients. Both clients are connected to the server (the server tunnel log displays both clients connected with some amount of traffic).
Server can ping client1, client1 can ping server.
Server cannot ping client2, client2 cannot ping server.
It gives me some error like "ping: sendmsg: Operation not permitted.".

Below a list with the associated IPs:

[Server]
eth0 -- 10.0.1.1 (connected to ADSL modem on location A)
eth1 -- 10.0.3.133
tun0 -- 10.0.3.167 (VPN tunnel)
tap0 -- 10.0.3.123

[Client 1]
eth0 -- 10.0.4.1 (connected to ADSL modem on location B)
eth1 -- 10.0.3.137
tun0 -- 10.0.3.172 (VPN tunnel)

[Client 2]
eth0 -- 10.0.5.10 (connected to ADSL modem on location C)
eth1 -- 10.0.5.1 (used for in-house internet gateway interface)
tun0 -- 10.0.3.200 (VPN tunnel)

I opened any iptable chains to permit connections on client2. That didnt help. What can I do to fix that?
Thanks for help.

What VPN software package are you using?
Which Linux distro are you using?

I have used OpenVPN in the past with excellent results, but there are others.

It's OpenVPN on debian etch.