You are using phpFox version earlier than 4.6.0? Please refer instruction for this article here.
If you find that your Register or Login buttons won't open a popup after selecting, and you have your site set with www in the domain, this tutorial should help.
- Click Settings and choose URL Match.
- Click Enable and then Clear Cache.
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: