adding server in untrusted domain with firewalling


I want to add a managed SQL Server in a untrusted Domain. Between both domains a firewall is active.
A Connection test at the end of "Add Servers" wizard fails.

I can ping the target machine from diagnostic Manager Server, tcp port is open. MSMS on diagnostic Manager server connects successfully to target machine.
Used SQL Login has sysadmin rights. Anything else I missed to configure for successfully manage this Server?
Are there any logfiles written while connection test?

thanks a lot