This can be caused by PCs or laptops physically connecting to the network that are members of a workgroup and not on the domain. Compare the domain listed in the event log error with the "domains" listed in My Network Places (My Network Neighborhood | Entire Network | Microsoft Windows Network). This error will go away once the PC or laptop is off the network long enough.
Check the Description field for the specific error. This response is specifically for "0x1558000: The Windows system call NetSessionEnum returned an error for server domain_name\domain_controller_name. Error Code: 53 : The network path was not found."
Websense looks to its C:\Program Files\Websense\bin\dc_config.txt file to find domain controllers. Our file still had entries for old DCs. I deleted them from the file restarted the agent and the above event log error regarding internal domain controllers is gone.
See the link to "Websense Support Article #: 1077" for documentation about troubleshooting DC Agent.