For troubleshooting purposes, enable tracing of commands sent to Exchange Servers, reproduce the error and send us (support[at]adaxes.com) the log file. For information on how to enable the tracing, see the following help article: https://www.adaxes.com/help/?HowDoI.Per ... uests.html.
Also, you can check the logs on your Exchange server for related errors/warnings. If you are not sure, you can send us (support[at]adaxes.com) the logs for review.
Adaxes performs all operations in a managed domain using the credentials of the account that was specified during the domain registration. Could you try to create a remote PowerShell session to your Exchange server from the computer where Adaxes service is installed using credentials of the account? For information on how to check the account, see https://www.adaxes.com/help/?HowDoI.Man ... nInfo.html. If the connection establishes successfully, try to execute the Get-Mailbox cmdlet for one of the users for which the error occurred in Adaxes Web Interface.