The NS logs are flooded with errors regarding encryption keys and client computer guids
ITMS 7.6 and later
Data encryption for resource was requested, but encryption key is missing: COMPUTER-GUID-GUID-GUID-GUIDGUIDGUID
Unable to process session message: COMPUTER-GUID-GUID-GUID-GUIDGUIDGUID
<msg type="EndpointTrustInfo" guid="6939ee4b-2493-434f-9bd6-b077a224c870" />
Failed to load agent encryption key.
COM Exception errcode: 0x80077000
Possible corruption of the Task Server
In one case this was resolved by uninstalling and reinstalling (or upgrading) Task Service on the Site Servers the problem computers were assigned to. The problem computers can be identified GUID's shown in the errors.
As well there are NS log entries showing the IP address of such machines, like this one: