Oouch

When I use a************.oouch.htb (The Simple and Secure…) I always get an error:
Please enter a correct username and password. Note that both fields may be case-sensitive.
Of course I created an account and I am sure my username and password are correct. There are often 500 server error, but when I was registering user it went alright. I wonder if I miss something or the machine is that unstable?