Our users are receiving the following error when attempting to convert a lead, and SF support has told us it is due to a managed package yet could not identify which. Does anyone have any insight into what package may be causing this, and how to resolve? 

'Error: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, null: [] Class.leadconvert.BulkLeadConvert.handleRegularContactInserts: line 326, column 1 ' 

Any help is much appreciated.

Best,
Julian