My small brain says there's two things you can possibly look into:
- Make sure there are no firewall restrictions preventing the connection. E.g ModSecurity is known to cause issues.
ModSecurity is an Apache module which works as a web application firewall.
It blocks known exploits and provides protection from a range of attacks against web applications. However, sometimes, ModSecurity may incorrectly determine that a certain request is malicious, while it is actually legitimate. In such a situation we can white-list triggered mod_security rule on the server, so that you can bypass the block.
We have found and whitelisted a few triggered rules on the server. You can test your website updating once again now.
- There's a small chance an extension could be causing the issue too. Try disabling all modifications and see if your new members are able register at normal speed.
