is there anyway that the developper of this plugin take a look at my issue ?
Developers always busy, they might be able help you but you need wait patiently. And, I don't think the problem is caused by this Better WP Security plugin. And it seems not also the caching plugins (W3TC nor WP Super Cache).
what "referrer" is ?
Basically, you can think referrer (in the logs) means which page cause the error.
I may not solve you problem, but you should try. Do you want a test? Here I describe some technical things before we start:
404 error happens when a visitor or hacker or bot (google, bing, etc) visiting your page and requesting something (a file for example), but the thing requested is not exist.
404 error often being associated with hacking attempts, because hackers need to scan your website to find weakness in your website. So they trigger 404 errors.
Sometimes, improper configuration may cause 404 error too, especially caching plugin. Because they change the way how your website retrieve and provide the resource (the file) to visitors.
Okay, now do you want to do the test? As you said, it will be better perform on your website rather than I build a test site.