impossible to upload files to site with ftp client

My site got hacked. I have been trying many things to resolve the issue. (I already cleaned 404.php and functions.php which both contained malware). Now I wanted to change passwords. After changing the password of the database in the PhpMyAdmin of my host (www.linuxsystems.be), I wanted to adjust the password in wp-config.php, using FileZilla. But I was not able to change the file. I could see all the maps, but could not edit them. Finally I got the glorieus idea to download wp-config, edit it, delete the original one, and upload the editted one. Unfortunately, that didn't work. I am not able to upload it which resulted in my site not working anymore. I tried another ftp client (coreftp), but that one also didn't do it. First I thought it was an issue with my router or Firewall, so I tried every setting, but nothing worked. Finaly I asked a friend webdesigner (I am just an amateur) to upload my wp-config.php from his workplace. He is also using FileZilla and has clients which are also hosted by Linuxsystems. But also he couldn't upload anything on my site. So the problem is not with my router/Firewall settings. Now I really don't know what to do anymore and turn to you as my final hope. I hope you can help me. Kind regards, Mark