In the past here is what I have seen to cause a failure to connect:
Wrong IP
Incorrectly IP or subnet settings on admin PC
Duplicate IP
(since recieving a ping reply while directly connected, I would assume the above don't apply)
Bad NIC (usually I have seen on older ACTU1A processors)
Admin locked up (typically when using a Stratagy ES voicemail, and corrected by a cycling power)
Network emanager pre-install requirements not installed (But they are now installed with the emanager installation)
Broken NEM install (usually results in html, ASP, or other IIS type errors.)
Wrong community name, or default was changed on the CIX.