I have the same issue. The node is up with no problems, but the APM v.3.5 shows its down.
Tcupal,
If your node status is down that means that your ICMP packets are not making it to there destination. ICMP is how the node is shown to be up or down.
The reason that you are getting information from interfaces is because that information comes from SNMP.
I'd suggest trying to see if these packets are being dropped for some reason.