Microsoft is currently investigating an issue with Microsoft 365 which is resulting in multiple services being unavailable.

We're investigating an issue affecting access to multiple Microsoft 365 services. We're working to identify the full impact and will provide more information shortly.

— Microsoft 365 Status (@MSFT365Status) September 28, 2020

A look at Downdetector suggests most of Microsoft 365′ services are having issues, from Teams for Azure.

In their advisory Microsoft notes:

Title: Can’t access Microsoft 365 services

User Impact: Users may be unable to access multiple Microsoft 365 services.

Current status: We’re investigating a potential issue and checking for impact to your organization. We’ll provide an update within 30 minutes.

DownDetector’s map suggests the issue is worldwide, though of course Europe is sleeping at present.

Microsoft suggests checking out your admin portal for more details, but at the time of writing, even this was not functional.

Update: The problem appears to be mainly about logging into to services. Microsoft notes:

Existing customer sessions are not impacted and any user who is logged in to an existing session would be able to continue their sessions. Additionally, Power Platform and PowerApps properties are affected by this incident.

Microsoft says they have reverted the change they believe has caused the issue, saying:

Current status: We’ve completed reverting the change which was likely causing impact and are monitoring the environment to ensure that services are recovering. Scope of impact: Any user may experience access problems for Microsoft 365 services.

Unfortunately for me the issue still persists:

Microsoft appears to concur, saying:

We're not observing an increase in successful connections after rolling back a recent change. We're working to evaluate additional mitigation solutions while we investigate the root cause. Please visit https://t.co/AEUj8uAGXl for additional information on this issue.

— Microsoft 365 Status (@MSFT365Status) September 28, 2020

Update 2: The issue  was resolved after more than 5 hours of downtime, with Microsoft writing:

After extended monitoring of our service, we have confirmed that the issue has been fully mitigated and is running within optimal service levels.

Start time: Monday, September 28, 2020, 10:25 PM (9:25 PM UTC), End time: Tuesday, September 29, 2020, 3:25 AM (2:25 AM UTC)

Microsoft explained the issue as such:

A code issue caused a portion of our infrastructure to experience delays processing authentication requests, which prevented users from being able to access multiple M365 services.

Microsoft says they will be reviewing their code to understand what caused the code to stop processing authentication requests in a timely fashion.