UPDATE: Azure & Office 365 login issue (multi-factor authentication)
Updated: May 3, 2019
Actually, there are some issues when users want to connect to Azure and Office 365 resources.
If they have multi-factor authentication enabled, they aren't able to login, because Microsoft has a bigger problem with the service, as they write on the status pages.
As Microsoft writes on the status pages, Engineers are working and deploying some hotfixes. This will take some time to synchronize between all the systems and regions.
Conclusion: You wanted to have your Azure/Office 365 secure. What is more secure than no login possibility? (take it with humor - it's Monday!)
UPDATE Azure (20.11.2018, 12.30 CET)
Login with multi-factor authentication to Azure resources is working again.
As you can read at the Azure status page, the root cause was initiated by requests from MFA servers to Redis Cache in Europe causing latency and timeouts. After fail over the traffic to North America a secondary issue caused, where servers became unhealthy and traffic was throttled to handle increased demand.
As you only have 60 seconds to accept the second factor, latency isn't your friend.
Microsoft Engineers have deployed a hotfix to eliminate the connection between Azure Identity Multi-Factor Authentication Service and a backend service. They also "rebooted" the impacted servers to get it up again.
UPDATE Office 365 (20.11.2018, 12.30 CET)
The issue with multi-factor still persists.
The self-service for password reset is also impacted.
Root cause is an update of the MFA service which introduced a coding issue and prevents users from being able to sign in to Office 365 services when using multi-factor authentication.