Hi Rui,
Was digging around internally and found one other rare case where they had multiple domains involved.
But if you are accessing Pro from the same Enterprise based deployment (Enterprise portal) then this shouldn't be the issue. I believe we might have a Web Adaptor or Portal configuration issue here just because of the redirect issue you mentioned previously.
Quick Test:
1. On the same machine as the Enterprise portal and ArcGIS Pro, can you go to your Internet Explorer, open the portal home page ie https://domain.com/portal and this should automatically log you in, as your AD (active directory account) since you have IWA configured, once logged in, Open ArcGIS Pro and see if you are still presented the same issue (assuming that the "Active Portal connection" is still set to https://domain.com/portal, or are you prompted for a login?
2. On your Web Adaptor IIS, can you temporarily set the Anonymous Authentication to enabled while having Windows Authentication enabled as well. (again I know this not standard practice, just wanted to rule out something here) if this does nothing, then set it back accordingly.
Use Integrated Windows Authentication with your portal—Portal for ArcGIS (10.7 and 10.7.1) | ArcGIS ...
Additional questions:
-Do you have a Web Context URL set for portal? if so, is it the DNS name?
Configure your portal to use a reverse proxy server—Portal for ArcGIS (10.6) | ArcGIS Enterprise
-Do you have ArcGIS Solution Deployment add-in installed?
Install the ArcGIS Solutions Deployment Add-In—Get Started | ArcGIS Solutions for Water
-Any Proxy involved here?
How To Check the Proxy Server Settings on Your Computer