Allow Comments is enabled on the Event Action record. When submitting the Event for approval, ???Comment_vod__c-??? displays in the Comment box:
The user does not have Read access on the EM_Event_History_vod object.
According to the field definition of EM_Event_Action_vod__c.Allow_Comments_vod__c:
If checked, clicking this button will prompt the user to enter in comments. This comment will be stored along with the action in the Event History object. The user must also have edit permission on the Comments field in the Event History object for the comment dialog to display.
Since the user does not have Read access on Event History, getting the error in any event actions allows comments.
CRM Help Documentation: