FTP unreliable and no error log to diagnose?

You're not the first to run into this. FTP is actually older than the Internet, and the protocol design is entirely at odds with modern network security.

There's a second and parallel connection involved with FTP, and the port selected for that is only known to a firewall through deep packet inspection — the firewall has to sniff the FTP traffic — or the firewall can use a 'trigger' to open the ephemeral range when an FTP connection arrives. TCP 21 is just a control port, it's the data port connection that's the usual problem — no sane firewall would ever allow such a connection, hence the problems that commonly arises with FTP. Not all firewalls support all these features, either.

Easier, just get rid of your firewalls and you'll have no problems with FTP. If you can't do that, then open the entire ephemeral port range on either the client or the server firewall or preferably both, or configure and use deep packet inspection or a trigger if your firewall supports one or both.

Or even easier — and far more secure, as FTP transmits your login credentials in cleartext for anyone with a privileged network position to monitor — switch to sftp. sftp shares three letters and its basic purpose with FTP, but is otherwise a vastly better and more secure tool, and far easier to configure with modern network security. Open TCP 22 (ssh) and you're done. You can also set up certificates with sftp, which means you can have far better passwords, access revocation, per-user credentials that you issue, and other benefits.

Some more details on FTP, ephemeral ports, active and passive transfers, etc...

sftp is built on ssh, and ssh servers are available in both OS X client and OS X Server. ssh is the remote login protocol.

For something similar to what you're doing, I'd typically set up the person you're exchanging files with as a user on the Mac you're hosting files on, and either a password-based login or I'd generate a digital certificate and use that. Or I'd set up a virtual host on Rackspace or Azure or Linode or another provider, or some Amazon S3 storage, and push the files you want to share up to that.

I have some OS X and Windows client documentation, but not OS X server documentation. That documentation is for ssh, but sftp — being based on ssh — uses the same certificates and the same setup.

Console.app has no log entries for FTP? What I recall of that, the entries went into the system log. There are other discussions that might be useful here, though I'm not running an ftp daemon on any of the local OS X Server boxes to confirm the settings.