Strange website behaviour - GET parameter with a specific value is causing the website to redirect to 403
Almost every time I've found this strange behaviour there was some sort of security mod in the server, either at web server level or at PHP level. Hosting providers are often not even aware of it, perhaps because it's a default component of their control panel.
Your only chance is probably to find out what exact module there is and read the documentation to determine whether you can change the settings.