This article provides brief information on the following error appearing within your SQL logs on a monitored SQL server despite the application monitor being able to poll the server without issues:
20151022 11:30:46.70 Logon Error: 18456,
Severity: 14, State: 6.
20151022 11:30:46.70 Logon Login failed for
user 'DOMAIN\USERNAME'. Reason: Attempting to use an NT account name with SQL
Any SQL server monitored by SAM
The reason for the errors is SAM will attempt log into the SQL server via both Windows credentials and SQL credentials. This is by design and will cause a login failure for the version of authentication that it is not using.