Cause:
If node has some IP address in a subnet that already exist in NetXMS (in Entire Network)
and node's netmask does not match subnet's size NetXMS will report that node
has invalid netmask on this interface and change node's status to 2.

Solutions:
So to get rid of this message any of the following can be done:

1. login to this node and fix netmask on node's interface to match existing network definition (and netmasks of other nodes in same subnet if there are some) and poll configuration.
2. delete "Subnet" object and poll configuration from node, to recreate "subnet" object with correct network size.

Option 2 worked for me. Thanx!