In one scenario, the AppMonitor legacy Agent 5.5.4 failed to start collecting data. The AppMonitor Agent log has the following error,
"Agent exited with status 1"
All the OS Agents (Linux_System, DiskIO, Process, etc) are collecting data. Reinstalling the agent or the fglam did not help.
Missing localhost entry in the host's /etc/hosts file.
Placing the fglam in debug2 indicated:
Unable to gethostbyname for localhost.
Unable to connect...
Then, reviewing the linux-system-info.txt ==>
::1 localhost6.localdomain6 localhost6
That meant: No localhost defined.
Edit /etc/hosts to include localhost entry
::1 localhost localhost6.localdomain6 localhost6