OK, you answered half of my question :)
The other half is where is the subdomain's web root. If it's inside the web root of the ville-draguignan.fr site then you will have problems indeed: the .htaccess files are inherited based on the filesystem (where the files are in the server). The subdomain in this case would inherit the .htaccess of the main domain which would cause all sorts of problems, leading to an error trying to load the page which would make Joomla try to load an error page which, if missing from your template, would result exactly in the error you said you have.
That's why I need the answer to the other half of my question.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!