From inside the server I can connect to the outside with ssh as root. Strangely enough from the outside it is impossible to enter that server with both ssh or telnet as root. It's like root has no permission to enter that particular server.

Everytime I try to connect the following error occures: Received signal 11. (no core). The authentication however does succeed.

Here's the output of using ssh -v:

warning: Development-time debugging not compiled in.
warning: To enable, configure with --enable-debug and recompile.
debug: Ssh2/ssh2.c:1848: User config file not found, using defaults. (Looked for '//.ssh2/ssh2_config')
debug: Connecting to server1, port 22... (SOCKS not used)
debug: client supports 3 auth methods: 'publickey,keyboard-interactive,password'
debug: Remote version: SSH-2.0-4.0.5.5 SSH Secure Shell
debug: Major: 4 Minor: 0 Revision: 5
debug: SshProtoTransport/trcommon.c:1464: lang s to c: `', lang c to s: `'
debug: SshProtoTransport/trcommon.c:1530: c_to_s: cipher aes128-cbc, mac hmac-sha1, compression none
debug: SshProtoTransport/trcommon.c:1533: s_to_c: cipher aes128-cbc, mac hmac-sha1, compression none
debug: Remote host key found from database.
debug: server offers auth methods 'publickey,password'.
debug: Ssh2AuthPubKeyClient/authc-pubkey.c:1733: Starting pubkey auth...
debug: SshUnixUserFiles/sshunixuserfiles.c:354: Using '//.ssh2/identification' as identity file.
debug: Constructing and sending signature in publickey authentication.
debug: Ssh2AuthPubKeyClient/authc-pubkey.c:725: reading private key //.ssh2/id_server1
debug: Ssh2AuthPubKeyClient/authc-pubkey.c:1822: Public key authentication was successful.
Authentication successful.
Received signal 11. (no core)
debug: Got session close with exit_status=255
Connection to server1 closed.

And yes it's fine if you export it to the AIX group.

Thanks!