Authentication is not working for all users. They are receiving the message, "Invalid Synchronous Response" when trying to authenticate with their Defender token.
1. Check the "Token Violation" count and reset if necessary:
Using the Defender MMC (ADUC) navigate to the user's "Properties", select the "Defender" tab. Use the "Test" button to verify the token is valid.
2. Reset the "Violation Count" by clicking "Reset"
3. Check the DSS logs for any messages related to the user's authentication attempt. If you see something like, "authentication abandoned user test1 Session ID: 34C4096D" then it's likely that Defender is requesting a token response, but is not receiving one. Thus the attempt is abandoned. For the user this will appear as "Invalid Synchronous Response" when they try to authenticate.
For more information on gathering DSS logs, please refer to Knowledge Article 124744, Locating the Defender Security Server Audit Log.