My new iMac is seen on the network but can't be accessed.

I got the latest 27'' iMac, works perfect except for the file sharing.

I installed this Mac by using the automatic copy of files etc from my Macbook Pro.

So here are my issues:

- I have file sharing activated (both ADP and SMB on my user account), with read access set for "everyone" on 3 different folders, and read/write for my own user account. My other mac sees the iMac, but the connection fails immediately. If I use "connect with user..." and enter my own credentials, I do see all of my drives. But the guest user does not seem to be working. I have sharing activated on the guest user account though.

- I have a raspberry pi on my network, which appears in the Finder of my iMac. But when I click on it, the connection fails immediately. It works from my other Mac, without any configuration (anonymous logon works).

I searched the whole web for similar cases, but could not find any. I suspect an issue in Mountain Lion because my whole configuration is the same as on my Macbook Pro under Snow Leopard and this one works perfectly fine.