When you try to get data you get the following error:
Cannot retrieve the data. Unexpected server error. View Director server event logs for further information (Refer Citrix KB article CTX130320).

If you dive deeper into the Event logs in Applications, you'll find this error as well:
An unexpected error occurred when accessing the data source 'Unknown error.' ('http://<servername>/Citrix/Monitor/OData/v3').

User: '<username>'
Console operation: 'Preview Custom report query response'

Additional information:
'Unexpected character encountered while parsing value: <. Path '', line 0, position 0.'

I had a day when users were unable to start their VDI instance and when I tried to check the VM's XenCenter refuses to connect as all. 
You could ping the server fine and even access it by SSH with no issue.

When you try to use application aware backups in Veeam with the Hyper-v hypervisor on Ubuntu servers you will notice it doesn't work and will always give you a Failure. This is because the integration service for Linux isn't installed by default unlike Windows machines. 

If you google this error, you will find out that there are a whole bunch of reasons why this error happens. The only way to find out why it gives that error is by going into the transport logs for your frontend.