I can see other Macs on network, but I can't connect via afp, smb or Bonjour

Hello.

I have a Mac Pro running 10.6.8, a Macbook Pro running Lion, an Ipad 2nd generation and an Iphone 4. All the devices "see" each other, but I am unable to establish a connection.

For instance: I can see the other mac in the finder, but a connection can't be made neither via clicking or using command + K

I can see my iTunes libraries in the remote app, but it wont connect to the iTunes library. Same with iChat: in the Bonjour network both macs see each other, but no message is delivered.

However, I can connect to the harddrive, that is connected to my airport extreme. Previously I thought my router was the problem, but now I exchanged it with this airport Extreme.

In another thread I read something about adding 10.0.1.* to "Bypass proxy settings for these Hosts and Domains" in the network proxy settings, but this did not help. Although I am not sure if I entered it right.

Any ideas how to resolve this problem?