Since introduction of MultipleChoicesField and SelectObjectField value checks with b71cd218, when a portal user was trying to do an action on a resolved UR the agent_id field was rejected. The reason for the rejection is the corresponding SelectObjectField query is containing the portal user scopes (query generated in ObjectFormManager::Build), and of course the agent_id set isn't in this scope...
As a workaround we are disabling those checks for read only fields (values are set server side and possibly in a different context than the portal user)