If your customer is using an Exchange Server OnPrem, then out of the box Exchange Synchronization via web services does not work.

Repro:
- setup Exchange Server OnPrem
- populate the Office 365 Authentication mail address with the E-mail address that is hosted on the local Exchange Server
- press Validation Exchange Connection via Process in Exchange Sync. Setup.
Fiddler does show it tries to connect to Exchange Online. There are partners / customers that do own a local Exchange Server OnPrem.
In Exchange 2018, Dynamics 365 BC up to release v14, it is easy to update the following code:
VAR
Service@1000 : DotNet "'Microsoft.Dynamics.Nav.EwsWrapper, Version=11.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.Microsoft.Dynamics.Nav.Exchange.ExchangeServiceWrapper";
ExpiredTokenErr@1003 : TextConst 'ENU=Trying to reconnect. Please close and reopen the add-in.';
To:
VAR
Service@1000 : DotNet "'Microsoft.Dynamics.Nav.EwsWrapper, Version=11.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.Microsoft.Dynamics.Nav.Exchange.ExchangeServiceWrapper";
ProdEndpointTxt@1002 : TextConst '@@@={Locked};ENU=exchange.local/.../Exchange.asmx';
ExpiredTokenErr@1003 : TextConst 'ENU=Trying to reconnect. Please close and reopen the add-in.';
For BC150, BC160 and later we added an event you can subscribe to. The code adjustment (not a platform fix) will be part of July 2020 CU.
Thanks.