If you find that your Register or Login buttons won't open, and you have your site set with www in the domain, this tutorial should help. 

Another solution is posted in this tutorial for log in issues.

If the above doesn't help, here are a few threads from our community:

https://community.phpfox.com/forum/thread/53970/login-not-working-v4-2/?view=332727

https://community.phpfox.com/forum/thread/52764/short-url-problem-solved/