Enable second OWA site with form based authentication

Hi,

I'm performing a configuration for a 2nd OWA site, that will use form based authentication.
The default owa site uses integrated authentication.

I've created a second site, and used new-owavirtualdirectory and new-ecpvirtualdirectory to create the OWA and ECP VDirs in the new site.
I've made binding on port 999 (https)

I then copied the OWA and ECP directories located in H:\Exchange Server\V15\FrontEnd\HttpProxy (the ones from the first site), and renamed them owa2 and ecp2

I then changed the paths of OWA and ECP on the new site to these locations.

When I then configure basic authentication, everything is working as expected (prompt > insert credentials > OK), but when I use form based, I get redirected to https://owa.customer.com/owa/auth.owa, and then receive a "bad request" error after logging in via the form.

default site : https://owa.customer.com/owa/
newly created site : https://owa.customer.com:999/owa/