Hi Jeremie,
An Ftp error 550 occurs when the user does not have the required permission to access the server or when no such file/folder/directory etc exists in the server that you are trying to access.
Can you please try and connect to your Ftp server using any third party service like Filezilla, Core ftp etc just to confirm that you have the required permissions and the Ftp credentials are correct.
Please do let me know if you were able to connect using Filezilla or not.
Regards,
Rohit Nair