Error: HTTP Error 400 - when clicking the Unsubscribe link in Approved Email of CRM
The integration user on the Approved Email Administration page has no Field Level Security (FLS) access on the field Language_vod__c in Account Object.
Grant the integration user on the Approved Email Administration page FLS access on the Language_vod__c field in the Account Object.
Please refer to this guide about FLS if needed: Field-Level Security.
CRM Help Documentation: Configuring Approved Email Admins and Integration Users