I think I encounter the same issue, I am absolutely sure of the credential, I can use them to login on the page from anywhere. Same issue => https://kb.paessler.com/en/topic/26533-is-there-a-workaround-for-this-error-http-1-1-401-unauthorized
There is a form to login to be granted to the page.
I tried to change the method to POST as mentioned, I tried to change SNI Inheritance, Protocol Version, I tried to add the credential with domain\username and only username.
I am trying the HTTP (with special cred in device) and HTTP Advanced, and also HTTP Data Advanced.
This is the result's page of APP-V (Microsoft Application Virtualization):
This XML file does not appear to have any style information associated with it. The document tree is shown below.
<Package PackageId="9abf2a94-dc7c-4abf-8897-bf425b0662d4" VersionId="5ed07d43-4ae7-481f-8e75-ffb87c8d63a5" PackageUrl="
fednet.local\domdata\APPV\MindManager 9\MindManager 9.appv">
<DeploymentConfiguration ConfigurationId="219" Path="policies_2494/30/219" Timestamp="2013-07-10T08:36:28.203"/>
<UserConfiguration ConfigurationId="40" Conflict="false" Path="policies_2494/30/40" Timestamp="2013-01-08T13:08:41.917"/>
<Package PackageId="6339018a-896a-44ea-8231-68c98f9dae7f" VersionId="87e3ab54-6cbd-4c2b-9be5-f6260611b133" PackageUrl="
fednet.local\domdata\APPV\Odyscube V7.2-1\Odyscube V7.2.appv">
<UserConfiguration ConfigurationId="1121" Conflict="false" Path="policies_2494/397/1121" Timestamp="2017-02-14T14:34:22.763"/>