[coldark]

First and most obvious to me is where is your Smart Centre's Default Gateway Configured? I suspect it is set to IP390_1 Internal NIC. If that is so, then when you ping the EXT NIC of IP390_2 it will route there via IP390_1, and the traffic from your ManagementServer will appear to be arriving at the External Address - but claiming to be sourced from an Internal Address - a classic AntiSpoof situation. If this IS the case, what you should do is set the SmartCenter's Default Gateway to the Internal Cluster IP, and put 2 routes on your SmartCenter:

1) Traffic to IP390_1 External via IP390_1 Internal <======== Allows Management to communicate direct with IP390_1
2) Traffic to IP390_2 External via IP390_2 Internal <======== Allows Management to communicate direct with IP390_2

IF THE ABOVE IS NOT THE CASE - then...

Any chance you could screenshot the Topology Tab of your Cluster Object.

In particular, I mean SmartDashboard go to your Cluster Object > Topology > Edit Topology - take screenshot.

As an aside, and nothing to do with your original question - the two sync nets you have shouldn't normally be specified as Cluster interfaces - I think it is more normal to specify these only as sync interfaces (1st Sync and 2nd Sync).

What type of clustering are you using? VRRP?