@baris d'oh! I could have sworn that I'd updated them all to latest.
In nodeBB version 1.18, users get 504 error when trying to register.
Go to the register page.
Choose a username
Enter your email address on the next page and wait. The result is 504.
Are you using the registration notification plugin? It seems to be causing issues for some people
@pitaj yes, I had to uninstall this yesterday due to the same reason.