Wow…I’ve never heard of that method being used, actually, so I’m posting here to watch updates on the thread.
My first (educated guess) is that the Web Reports auth method probably only works for “local” Web Reports accounts, where the username and password are stored in the web Reports database itself. If Web Reports is using Active Directory / LDAP / SAML back-end, you’ll probably need to configure Inventory to also use the same back-end, and not proxy the auth through Web Reports.
It’s probably worth opening a support incident to validate though.