I ran into this error on a client’s system when trying to convert a lead to a contact. This area had not been modified, and worse, it was only happening some of the time. I finally was able to track down the issue by utilizing Reflector and analyzing what the SaveLeadHistory method was doing (this file is in the Sage Business Rules dll).
It turns out that the SaveLeadHistory attempts to create several history records and other things in the new contact record. One of these additional things is to create a lead source on the contact corresponding to the lead source of the lead.
The error was occurring because some leads had been assigned a lead source that was no longer in the system. The LEAD table held a LEADSOURCEID that was no longer valid. When the SaveLeadHistory method was running, the SalesLogix entity logic was attempting to find the related Lead entity that did not exist.