FileZilla Hangs Initializing TLS

Question

I've read the other questions about this problem, but the only solution that works is insecure FTP. After spending two months cleaning up after hackers this summer -- and not realizing it WAS insecure -- that's not an option I'm wanting to implement.

Status: Connection established, waiting for welcome message...
Response:   220---------- Welcome to Pure-FTPd [privsep] [TLS] ---------- 
Response:   220-You are user number 1 of 50 allowed.
Response:   220-Local time is now 16:13. Server port: 21.
Response:   220-This is a private system - No anonymous login
Response:   220-IPv6 connections are also welcome on this server.
Response:   220 You will be disconnected after 15 minutes of inactivity.
Command:    AUTH TLS
Response:   234 AUTH TLS OK.
Status: Initializing TLS...

There is no time-out. It just hangs there. FileZilla insists is a server configuration problem, but I did have it working at one point. I upgraded this weekend to v. 3.24.0 and, once again, it's not working. I've tried backtracking to earlier versions, with no luck.

I ran it through ftptest.net and got this:

"Your server is working and assorted routers/firewalls have been correctly configured for explicit FTP over TLS as performed by this test. However there have been warnings about compatibility issues, not all users will be able to use your server. For maximum compatibility, consider resolving these warnings."

The only warning, early on, is:

"Warning: The entered address does not resolve to an IPv6 address."

And then the ftptest.net continues on successfully.

I've wasted a lot of time on this this weekend. I checked the FileZilla forums for similar problems and, of course, they are there, but the stock answer is that it's a problem with the server. Doesn't the fact that ftptest.net ran fine indicate that it's NOT the server? Also, as I said, it was working fine until I upgraded to the new version.