(401) "Unauthorized" - error after CU update on Exchange.

The remote server returned an error: (401) Unauthorized

The above problem occurs when the client is installed directly on the Exchange server.

After you have installed the CU 22 for Exchange 2013, CU12 for Exchange 2016 or CU1 or higher for Exchange 2019, you will receive the above message when accessing EWS.

How to fix it:

Try to use the IP address of the server or localhost. If that doesn't work, try the following.

You can enter the EWS address of your server into the following Registrykey: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0 as a MultiString value named DisableLoopBackCheck 

1. Open the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0 
2. Create a  "MultiString" value named DisableLoopBackCheck 
3. Enter the CNAMEN or DNS alias to your EWS into the value field.

Note: If using ExMixedFolders or EEAttachments, you get a error-notification into the log which contains the name you have to enter:

     - WWW-Authenticate:Basic realm="THIS IS THE VALUE",Negotiate,NTLM