I'd start at the Settings page and go to Windows Credentials and confirm his data isn't in there, most of the modules also have their own separate credential libraries so depending on what you have installed you could check under the different product specific settings pages to be sure.
Loop1 Systems: SolarWinds Training and Professional Services
I have seen this happen when trying to poll VMware. After you check the windows credentials click the virtual setting and check the VMware credentials library. I know someone just recently having that issue and VMware was the culprit.