'Need admin approval or ' or '404. The event contains errors. REST API is not yet supported for this mailbox'

Recent changes have happened with the way we add events to Outlook.com and Office 365. Before we used Microsofts "direct method" where you could set a few event parameters and directly direct the user to their Outlook.com or Office 365 calendar. However, Microsoft has made a mistake in a recent update resulting in the events being blank (without event details). So we had to switch over to the Microsoft Events API which works for all users. 

• Action: 
  When a user clicks either "Outlook.com" or "Office 365" in an "Add to Calendar" button below error might be triggered: 
• Error: 
  Error: 404. The event contains errors. REST API is not yet supported for this mailbox.

The issue is directly tied to the user's Office 365 accounts/install. When you, as an organization, buy and set up your Office 365 software you can turn things on-and-off. In order to interact with the Microsoft Events API the "Third-party app access" needs to be turned on. When that has been done our App at Microsoft can connect to the user's calendar (if the user has granted access). The setting can easily be turned on but it requires that the user contact their local Azure/Office 365 administrator to do so.

Below are some articles from Microsoft that describes the issue and what needs to be done for the administrator.

Overview article here:
https://docs.microsoft.com/en-us/office365/enterprise/integrated-apps-and-azure-ads

And more detailed here:
https://docs.microsoft.com/en-us/microsoft-365/admin/misc/integrated-apps?redirectSourcePath=%252farticle%252f7e453a40-66df-44ab-92a1-96786cb7fb34&view=o365-worldwide#__toc379982114

We're keeping a close eye on Microsoft to learn when/if they'll fix the issue. In case they fix it, we'll turn the old settings back on. For us, it was a matter of it not working for everyone, or not working for a few. We do prefer the "old" method and hope Microsoft will resolve the issue.