This sounds like a Joomla! issue, not an Akeeba Backup one - our code has not even run at this stage. Please try all of the instructions mentioned in our troubleshooting page: https://www.akeebabackup.com/documentation/troubleshooter/abinstallation.html
Please do follow the FTP instructions, even though you already have set it up on your site already, as I think that you have the wrong FTP directory set up in there.
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!