Divi theme is totally fine! … I’ve used it on wp install running discourse SSO and it worked absolutely fine.
the problem is rather something removing the discourse hook from the URL and rather redirecting the default register page to something else.!
I was able to recreate your issue by installing the theme my login plugin so I’d suggest You first try disabling it and then ensuring that the flow works fine! I hope simon can provide better help at resolving the compatibility between two plugins.