When I rename main.php to main-disable.php everything is fine. I've added my IP address to the WAF.
I've not experienced this previously, but I have the website built on a temporary URL (http://xxx.xxx.xxx.xxx/~cpanelusername) via cPanel until I am ready to migrate the domain. Do you think that could be related to the issue I'm facing?
Renaming .htaccess makes no difference so I have ruled that out (I think).
My hosting is Siteground. Anything you can do to advise me would be much appreciated.