There have been no changes to this user in CRM. This is not an AD authorization issue, he can create new opportunities and update existing ones just fine.
Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=6.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: The user Id is invalid.Detail:
<OrganizationServiceFault xmlns:i="www.w3.org/.../XMLSchema-instance" xmlns="schemas.microsoft.com/.../Contracts">
<ErrorCode>-2147214049</ErrorCode>
<ErrorDetails xmlns:d2p1="schemas.datacontract.org/.../System.Collections.Generic" />
<Message>The user Id is invalid.</Message>
<Timestamp>2019-01-11T11:51:32.1914689Z</Timestamp>
<InnerFault i:nil="true" />
<TraceText>
[USD.CRM.Plugins: USD.CRM.Plugins.PostOpportunityCreate]
[4a31f3e4-a186-e611-80ea-005056801edc: PostOpportunityCreate]
Entered USD.CRM.Plugins.PostOpportunityCreate.Execute(), Correlation Id: 8840e165-d83f-47c0-a083-f3eaccf45320, Initiating User: 74962ae0-8280-e811-8108-005056801edc
USD.CRM.Plugins.PostOpportunityCreate is firing for Entity: opportunity, Message: Create, Correlation Id: 8840e165-d83f-47c0-a083-f3eaccf45320, Initiating User: 74962ae0-8280-e811-8108-005056801edc
Exception: System.ServiceModel.FaultException`1[Microsoft.Xrm.Sdk.OrganizationServiceFault]: The user Id is invalid. (Fault Detail is equal to Microsoft.Xrm.Sdk.OrganizationServiceFault)., Correlation Id: 8840e165-d83f-47c0-a083-f3eaccf45320, Initiating User: 74962ae0-8280-e811-8108-005056801edc
Exiting USD.CRM.Plugins.PostOpportunityCreate.Execute(), Correlation Id: 8840e165-d83f-47c0-a083-f3eaccf45320, Initiating User: 74962ae0-8280-e811-8108-005056801edc
[Microsoft.Crm.Sales.ObjectModel: Microsoft.Crm.Sales.ObjectModel.Connections.ConnectionPlugin]
[cd862d32-4564-49b5-b074-e1ab6e1f71ab: ConnectionPlugin]
</TraceText>
</OrganizationServiceFault>