I have seen this before but have never found the answer.
I have a 450i SM on the bench that I can only connect to (via the web interface) when it has a link up with an AP. If I have it stand alone on the bench with no AP I can not connect to it. This unit has been working onsite for years and I found the same problem onsite and at other sites I look after with the ePMP as well.
I also can not ping it.
I know the IP address and the VLAN etc. I can see it when running wireshark all ok.
Thanks