Setting up SSO: Whoops, looks like there was a problem. Please try again.

I'm trying to complete the challenge to Set Up Single Sign-On for Your Internal Users and I keep getting this message:

Whoops, looks like there was a problem. Please try again.

Things I've tried: 

• Clear the browsers cache -> Logout & login again then check challenge
• Check your challenge in incognito mode/Private Window or in a different browser
• Repeated it several times in brand new playgrounds

I've noticed that the #Trailhead shows that the recipient URL should have a format as shown below

but in my Single Sign-On settings the Login URL which If I understood well is the Recipient URL, is the following:

My Login URL doesn't seem to match the same format of recipient URL shown in the example, I suspect this could be the problem but i don't know why or how to fix it.

Thanks in advance