Loggedout page in IIS web site returns 404

I'm running PowerShell Universal 4.2.20 on IIS in a web site, i.e. it runs at https://mywebsite.com/Portal/
I'm running an App (https://mywebsite.com/Portal/myApp) and when a user logs out, he's redirected to https://mywebsite.com/loggedout. Of course this page does not exist as it should be https://mywebsite.com/Portal/loggedout

I could not find anywhere I could configure this (either loggedout path or that the website is not running on / but on /Portal).
What am I missing?