Symptoms

SharePoint Document 2010 event (Copy document) is throwing "Value cannot be null. Parameter name: g" error when we copy document from one library to another library.
 

Diagnoses

Issue is related to the specific SP site columns (User/Group column type). The following attributes are missing from custom site column definition when comparing to out-of-the box SP columns.
https://social.msdn.microsoft.com/Forums/sharepoint/en-US/0cb7925d-2d7b-4259-9788-b0774343fd4e/undocumented-attributes-about-site-column-definition?forum=sharepointdevelopmentlegacy

Sortable="FALSE"
List="UserInfo"
ShowField="ImnName"
UserSelectionScope="0"
EnforceUniqueValues="FALSE"
 

Resolution

This scenario was specific to custom SP columns as some of the custom columns were created manually. Issue was resolved by adding missing attributes to User/Group SP columns.