Hi Romain,
One of the plugin files seem to be missing from the server, it may have been deleted by accident. Please try to deactivate, delete and reinstall the plugin, that should resolve the problem.
If that does not help and the file is there, then there might mod_rewrite rule which matches the file path pattern and tries to redirect to an unknown location. It is rare, but usually originates from a misconfigured .htaccess file or a misconfigured permalink structure.