Xenforo 400 error when trying to access index.php, but admin.php works fine.

I'm having a very strange problem migrating a site into a fresh centminmod server (built yesterday). I'm getting a 400 error across multiple browsers and computers when trying to access index.php, but admin.php seems to work just fine.

I've checked and triple checked all the obvious things that I can think of:
* Added autoprotect-bypass files
* Permissions
* NGINX conf settings
* Xenforo file check
* Nothing in any error logs
* Downgraded NGINX
* Downgraded PHP

Exact error is as follows:

Code:

Error 400
We're sorry, but we could not fulfill your request for /index.php on this server.

An invalid request was received. If you are using a proxy server, bypass the proxy server or contact your proxy server administrator. This may also be caused by a bug in the Opera web browser.

What am I missing here?

 

Never mind. It was an issue with DragonByte Security. grrrrr

 

Almost certainly a bug. Otherwise, a browser 400 error is a terrible feature.