I have configured ACS forwarder in untrusted domain, following link below:
Warning event popped up on ACS forwarder agent.
Event Type: Warning
Event Source: AdtAgent
Event Category: None
Event ID: 4369
User: NT AUTHORITY\NETWORK SERVICE
Forwarder unsuccessfully tried to connect to the following collector(s):
FQDN.xxx.xxx:51909, status: 0x80090325 (TCP connect), source: registry
If the list of collectors is blank, then AdtAgent was unable to locate a collector.
Common reasons for this message are:
The machine(s) listed is not online
AdtServer is not running on the machine(s) listed
AdtServer on the machine(s) listed is not listening on the specified port
TCP connectivity to the AdtServer machine is blocked by firewall, IPSec, or other filtering mechanism
AdtServer on the machine(s) listed actively refused the connection (due to policy or current activity load)
For detailed failure information, enable trace logging using the TraceFlags registry key and examine the AdtAgent.log in the \temp subdirectory of the Windows directory.
Code 0x80090325 indicate issue with Authentication (certificates in my case).
As soon as I reimported certificates, communication occurred.