EWS with Oauth 2 - "SoapFault exception: An internal server error occurred. The operation failed."

We have a working PHP app using Basic Authentication and we migrated to Oauth 2 but we receive this error from EWS.

Uncaught SoapFault exception: [a:ErrorInternalServerError] An internal server error occurred. The operation failed.

Our app was implemented long time ago in PHP using php-ews library, but since Microsoft decided to disable Basic Authentication we planned to migrate to Oauth 2. and we updated the library to this one which supports the Modern Authentication.  
Also we generated the token using this api https://login.microsoftonline.com/{{tenant_id}}/oauth2/token and client_credentials flow since there is no user interaction.

We use Office 365 Exchange Online permissions with full_access_as_app and https://outlook.office365.com/.default scope to generate the token. We use X-AnchorMailbox header and impersonate soap header as well.

Do you have any clue why we are getting this error and how to fix it?