Desktop Director displays the error on the dashboard: Cannot retrieve the data.
Event viewer logs shows:

• Failed to connect to data source 'Unknown error'
• 'The remote server returned an error: (500) Internal Server Error.'

CDF trace captured on the Delivery Controller shows exception:

OData API Bad Request: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.Data.Entity.Core.EntityException: The underlying provider failed on Open. ---> System.Data.SqlClient.SqlException: Connecting to a mirrored SQL Server instance using the ApplicationIntent ReadOnly connection option is not supported.

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Make sure that the Monitor service is configured to use SQL Mirroring properly.

Open powershell on the DDC with Admin account.

Check current connection string using: Get-MonitorDatastore

Create a new connection string, example:
$x = "Server=SQLSERVER1\INSTANCE1;Failover Partner=SQLSERVER2\INSTANCE2;Initial Catalog=MONITOR_DB_NAME;Integrated Security=True"

Reset connection to NULL (Only Monitor):
Set-MonitorDBConnection -DataStore Monitor -DBConnection $null

Set connection string to the new one:
Set-MonitorDBConnection -DataStore Monitor -DBConnection $x

If you have multiple Delivery Controllers, perform this operation on one controller only. You can restart Monitor service on other controllers and it will pick up a new connection string for datastore: monitor.

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Problem Cause

If the database mirroring is configured on the SQL server and service is not configured properly - Monitor service will ApplicationIntent ReadOnly which is not allowed.