XMLRPC failed connection

Good morning.

Does prod check has requirements regarding the xmlrpc calls?
I succeeded to connection this great module to all my websites but one and I see no reason why not.
- htaccess protection? I put xmlrpc on the whitelist and it does not work even when htaccess is disabled,
- wrong key? I copy paste the one from a working site and still not working,
- wrong xmlrpc activation? I activate it several times, flush any possible cache.

So I am quite lost here

As the error is "Failed to retrieve settings form from remote site, please verify the given URL and try again!", it comes from

function _prod_monitor_retrieve_functions($url, $api_key, $msg = TRUE).,

Any idea? Thanks.

Julien.