ODK Collect submit not respecting alternative port number

I would like to report my recent observation with ODK Collect (1.23.3).
I have the Aggregate server (v2.0.3) installed in the DigitalOcean Ubuntu droplet (recommended installation) and everything works fine. But when I change the server port from default 443 to some different value (e.g. 4443) and change the server setting in the ODK Collect to xxx.yyy.zz:4443 then:

  • Downloading forms works OK, but
  • submitting completed forms fails with a connection error (trying to connect to xxx.yyy.zz:443 - the standard HTTPS port number)

Regards Vranoch