What is the setting of the authentication profile on that virtual web server? The trick I found when using authentication on the UTM was putting in the correct username so it matched the AD account (the UTM needs to be able to get data from the AD, not joined to the domain as such but needs to be able to authenticate against AD), and SharePoint needs the basic authentication allowed in IIS for the relevant application.
What is the setting of the authentication profile on that virtual web server? The trick I found when using authentication on the UTM was putting in the correct username so it matched the AD account (the UTM needs to be able to get data from the AD, not joined to the domain as such but needs to be able to authenticate against AD), and SharePoint needs the basic authentication allowed in IIS for the relevant application.