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Unfortunately PHP does not provide more details than what we convey back to you. It returns a boolean (true / false) indicating if the connection is successful. In case of an error it does not provide any information as to why the connection failed.

If you are using plain FTP (not SFTP or FTPS) the things you need to check are the hostname, port, username, password and initial directory.

You also need to check if your origin server is filtering out FTP traffic.

Further to that, on the target server you need to make sure that the connection is not blocked by the firewall (IPtables) rules, the FTP server is indeed running and accepting connections, the port matches what you gave Akeeba Backup and the username and password do have permissions.

Here's how I would approach it.

First, try using the same hostname, port, username and password in Cyberduck to ensure that I can, in fact, connect to the FTP server. If that fails the problem is in the target server.

If it connects, I'd check the initial directory. It's been a LONG time since I last used ISPconfig but IIRC it uses vsftpd with chroot-jailed users to their home folders so the initial directory should be / (a single forward slash). If it uses an FTP service without chroot-jailing you'd need to use a full filesystem patch such as /home/myuser

At this point I'd check the FTP logs on the target server. If it shows no traffic I'd be wondering if the source server can connect to any FTP server. So I'd try to use FTP credentials to a known good server in Akeeba Backup and see if the connection is successful. If it's successful the problem is on your target FTP server (firewall issue).

Once we have established that a connection can be made and the hostname, port, username, password and initial directory are correct I'd look into whether it's possible to upload anything with that connection. If not, it's a target FTP server configuration issue, probably wrong permissions or having set up the FTP server in read-only mode.