You can check what the Source ID for NPC is currently with this command:
mysql -P3308 netqosportal
select sourceid, sourcetype, consolename, consolehost, mode, deleted, enabled, removable from data_sources2;
output should be something like this:
NPC should be set to a sourceID of 0. If not, update it with this command:
Update data_sources2 set sourceID=0 where sourcetype=16;
Restart the NetQoS Device Manager Service, and the error should clear.