Hello,
check the url that is giving the error in the Portal Logs, you should see a request to the 7443 port url of the portal machine, that's where the problem is, the forward proxy does not know how to handle that request, You could try to cheat using the hosts file
Another solution that you could attempt is to do the following, on the machine where portal is installed:
1) Locate the catalina.properties in the folder "<Portal for ArcGIS installation location>\Portal\framework\runtime\tomcat\conf"
2) back it up, you never know, but note that this file will get overwritten by Portal when it starts
3) stop other ESRI products on the same machine (server, data store, ecc...), the aim is to have as fewer java.exe and javaw.exe processes as possible
4) edit the catalina file, usually at the bottom you will find the forward proxy settings
5) delete the proxy settings (should be 4 or 5 lines, remove them completely) ans save the file
6) open task manager and KILL every javasomething process you see
After a certain amount of time, Portal will see that his tomcat is not working, and it will start it again, without rewriting the file and hopefully getting rid of the forward proxy until the next reboot
If everything works as I hope, you should now be able to log into the portaladmin and remove the forward proxy
Let me know
Fra