EWS with oAuth "An internal server error occurred. The operation failed."

[MichaelDenmark]

Hi...

Since yesterday, my synchronization application starts throwing exceptions with the following error

"An internal server error occurred. The operation failed."

It only happen for mailboxes when I authenticatate with oAuth (CreateWithApplicationOptions).

Has anyone experienced the same and found a solution?

Tracing the call:

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">
<s:Header>
<Action s:mustUnderstand="1" xmlns="http://schemas.microsoft.com/ws/2005/05/addressing/none">*</Action>
</s:Header>
<s:Body>
<s:Fault>
<faultcode xmlns:a="http://schemas.microsoft.com/exchange/services/2006/types">a:ErrorInternalServerError</faultcode>
<faultstring xml:lang="en-US">An internal server error occurred. The operation failed.</faultstring>
<detail>
<e:ResponseCode xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">ErrorInternalServerError</e:ResponseCode>
<e:Message xmlns:e="http://schemas.microsoft.com/exchange/services/2006/errors">An internal server error occurred. The operation failed.</e:Message>
</detail>
</s:Fault>
</s:Body>
</s:Envelope>

[#2]

@prestonsbutler @Mikhail295 @MichaelDenmark 

We have a working 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, so we updated the library to this one which supports Oauth 2.
We generated the token using their 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 how to fix it? 

[#3]

@nsgsanna