AppInsight for SQL Server

I'm using SAM v6.1.1.

I've configured monitoring a node over SNMP.
Overall server monitoring working fine.
Then I tried to set monitoring for SQL Server instance which was automatically discovered on that node.
I click on instance and get prompt for credentials.I enter windows credentials and try to test them - but test always fails.

I've checked that the user have all required permissions on SQL Server which listed here:
SolarWinds Knowledge Base :: AppInsight for SQL Requirements and Permissions

Is there a way to find out what is wrong?