We experienced an outage on 10/3/18 that blocked Classroom lab users from connecting to their virtual machines.
If you have any classroom labs that were created on or before September 10, 2018, you may see that your lab users cannot connect to their virtual machines in your lab. Your lab users will see an error message on their virtual machines that says: "Error: Object reference not set to an instance of an object."
The issue has been fixed. Please let us know if you are still seeing any errors.