Probably Username defined in the workflow doesn't have the permission to impersonate the User to impersonate?
Hi , how is this going on your side?
We are running Exchange Hybrid. Once I created an Exchange Only account the error was resolved.
In my case, the user account I'm using to authenticate with Exchange Online has permission to impersonate every user, but I still get this cryptic error message:
Error when creating calendar item: Reason: The service is not able to generate a response for the request. Error:
Unable to create calendar item. Error: An internal server error occurred. The operation failed., Mailbox '.....'