Error when using specific word in Users forms

Good morning.

I am using Solspace User on my site for event registration.

I have a custom field for allergies and when users enter “Penicillin”, it gives the following server error :

Forbidden

You don’t have permission to access /fr/ on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Any ideas ?