If you are unable to log in to Redgate Monitor with your AD credentials, this will help you disable AD authentication.

You will need sufficient permissions to update the Redgate Monitor repository.

In SSMS, run the following query against the Redgate Monitor repository:

UPDATE [RedGateMonitor].[settings].[KeyValuePairs]
SET [RedGateMonitor].[settings].[KeyValuePairs].[KeyValue] = 0 
WHERE [RedGateMonitor].[settings].[KeyValuePairs].[KeyName] = 'ActiveDirectory-Enabled'

You will then need to restart the Redgate Monitor BaseMonitor. 

Related article: https://documentation.red-gate.com/sm/troubleshooting/error-messages/can-t-connect-to-active-directory-authentication-service