This is quite strange.
I have a web application that connected to DB1 in server1 and there is some ssrs reports from DB2 in Server2.
Domain report service account has content manager permission. application users stored in user table on DB1
reports working in report portal url and Report service URL
SSRS reports loading if I host the app as http://webserver.local/app
it popups username password box for ssrs reports if I host the site as http://app.webserver.local
with same application pool. Same files content.
other functionalities working fine regardless the hosting
Application and SSRS reports working fine on dev PC.
may be a small mistake. but difficult to figure out.