I'm not sure if the problem about login already solve, but I was also able to sign up without problem, no cookie refresh or installation of any app, I received the email confirmation via my email fast in less than a minute and now I'm using the Demo via BTC Trading and sees it have demo for USDT trading too. I also try checking different buttons and links that brings me to join the affiliate program.
Hopefully their login problem above is now solved.
I did not really experience any problems signing up/in in their site. I just didn't read the email confirmation after I signed up. There is the instruction to be able to log in and connect to the MetaTrader 5 app. You just need to set a password in the account dashboard. That's what the setup tutorials don't have.