Portal Connection in ArcGIS Pro not working

[RiccardoKlinger]

When I open the URL https://my_internal_URL.corp/portal or the webadaptor-url https://webadaptor.corp/portal I am able to login via the browser. everything works fine.
Yet if I try to login via ArcGIS Pro 2.9 to the same urls I am getting the notification, that the portal is available and the green checkmark but after a login with correct username and password I am still getting "Not signed in" text shown on the Portals overview page.
Also an attempt to use arcpy to login is unsuccessful:

import arcpy
arcpy.SignInToPortal(r"https://my_internal_URL.corp/portal", "my_cool_user", r"my_super_secret_password!")

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "D:\App\ArcGIS\Pro\Resources\ArcPy\arcpy\__init__.py", line 2494, in SignInToPortal
return _SignInToPortal(*args, **kwargs)
ValueError: Unable to connect ArcGIS Online.
</module></stdin>

The WEBContextURL is set to work for our Reverse-Proxy under https://my_reverse_proxy_URL.dmz.corp/portal

[ArchitSrivastava]

Hello @RiccardoKlinger ,

We can try to narrow down the cause of this preferably trying some steps and once we narrow it down could come up with a fix. Hence, I would like to request if you could please try/confirm the following:

• What is the Authentication in Portal for ArcGIS like ArcGIS Logins or Enterprise logins?
• Have you imported "Root", "Intermediate" and "CA" certificates to Portal?
• What happens if we try to connect ArcGIS Pro using "7443" URL e.g. https://portalmachine.domain.com:7443/arcgis
• What does it show in ArcGIS Pro Logs? This can be launched by pressing Ctrl+Alt+M while ArcGIS Pro is open
  • For more information on utilizing and analyzing refer to this Start Diagnostic Monitor  

Hopefully this helps you narrow down the issue.

-Archit