error pinging LAN

[LC]

Hi,

I have a router build using FC4. eth0 is connected to the modem, eth1 is 
connected to the switch. I use roaring penguine to connect to the internet 
with success. Windows boxes in the LAN are also connected to the switch to 
gain internet access via the router. I had been running this setup for 2 
weeks and suddently it failed. The connection to the internet can be 
established, but it seem that there are some errors in the LAN. If i try to 
ping local machine of the lan, it tells me "ping: sendmsg: Operation not 
permitted"

Can anyone tell me what happened?

best regards