Platypuschan

I tried every possible combination of configurations in the last two hours.
What reliably fixed it for me is this:

Creating a "Allow 0.0.0.0/0" rule in the Access List Tab
Deactivating "Satisfy Any" in ACL Tab
Create a NEW Proxy Host.
Deactivating HTST, HTST Subdomains and HTTP/2 in the Proxy Host Tab. Force SSL works for me when everything else is off.
And yes. It is crucial to first create the ACL and then create a NEW Proxy Host. Adding the ACL to an existing proxy host will result in 403 error, or in logging in having no effect and login window showing over and over again.

Note: When I misconfigured it i had to "clear the site data" in the browser for it to work again once it was configured right.