microsoft-graph - Error: ResourceNotFound - Message: Resource could not be discovered

I've created a web app with Microsoft Graph API and it has been completed/tested and everything is working as expected. This web app is aimed to create calendar events for our Office 365 users. Right now there are a total of 9 users that calendar events are created for and two of them are having issues.

This is the specific response I get from Microsoft Graph API if I'm creating a calendar event for a user that is having issues:

Message:

{
    "error": {
        "code": "ResourceNotFound",
        "message": "Resource could not be discovered.",
        "innerError": {
            "request-id": "4d6efba0-fce2-4abe-9f60-be3df8b55d6f",
            "date": "2018-08-29T15:26:58"
        }
    }
}

I know that everything I'm doing is correct as the other seven users have no issues so the issue must be with the office 365 account its self. I've looked into account settings/permissions and everything I've checked is identical to a user that is working.

One of the biggest issues I'm facing with this problem is I don't understand the error its self. When it says "ResourceNotFound" my guess is that it's saying that it can't find the user account even though it does exist.

My question is what should be the next steps be in order to find a resolution? Should I see if I can recreate the mailboxes and transfer the email from the broken account to a new account? Or should I continue to try to find the root problem?