In our case the Websense user account which was used for directory services had gotten locked out. Re-enabling the account resolved the error and allowed users to log in to Websense Manager using their network user IDs.
I observed this event on our Websense server when the account that Websense uses to carry out LDAP lookups had its password changed. For information on this problem, see Websense knowledgebase article 2721: Windows NT or Active Directory - Not filtering by User and Group (go to the Websense knowledgebase page and search for the specified article).
To resolve, as mentioned in the article listed above, Log into your Policy Server, click on Server -> Settings -> Directory Service. In the right hand pane highlight your Domain Forest and click Edit. Check the Credential information is correct and update the password if necessary.