Thread:Project:Support desk/403 error when trying to create a page with a specific word in the title

When I try and create a page with the word 'Penicillin' in the tile I get a 403 error: site.com/index.php?title=Penicillin&action=edit Forbidden You don't have permission to access /index.php on this server. Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I've never come across this before, other words seem to be fine in the title. Is this word blocked somewhere? How do I debug this?