[pure-ftpd] Problem accessing PureFTP via FTPES

My web site is hosted by Catalyst2 and updated via PureFTPd and the
FTPc client.  I've accessed the site for years via non-secure FTP,
but I thought it's time to access it via FTPS.  Unfortunately it
doesn't work - PureFTPD closes both the control and data connections
immediately after the data connection is opened securely.

OS is RISC OS 5.27 (07-Apr-20).  FTPc is 1.55 (16 Nov 2019).  FTPc
uses AcornSSL 1.05 (09 Sep 2019) mbedTLS 2.16.5.

Let me list the observations:

- Access in non-secure mode works fine.

- Access in secure mode (explicit, passive) works fine on the control
  connection; AUTH TLS negotiates TLS v1.2; USER, PASS, PBSZ 0, PROT P,
  PASV, all OK.  Open a secure data connection and send LIST; PureFTPd
  immediately closes both the control and data channels.

- I've written some test code, which as no commonality with FTPc but
  uses the same OS and AcornSSL module.  The symptoms are the same.

- If, in my test code, I open the data channel as non-secure and wait,
  PureFTPd also waits.  If I then upgrade the data connection to
  secure (TLS v1.2 too), PureFTPd immediately closes both the control
  and data channels.

- FTPC and my test code access a test site, test.rebex.net, securely
  with no problem.

I can't tell you what version of PureFTPd is in use on the Catalyst2
site, because, although PureFTPd signs on with several 220 messages,
it doesn't show a version.

The author of FTPc and I are now looking for help to diagnose what's
going wrong.  Wireshark traces can be made available.

David Higton