Is 'admin@' a valid AD account, and have access to browse AD users?
To clarify, at the spot where you are erroring out, it's looking for a valid AD account, not necessarily a SolarWinds account.
Can you create a non group (single AD account?)
I think this error is for the credentials you use to log into AD, ie: what's above the screenshot. Try putting in your credentials as "ADMIN\domain" or try different credentials like your own.