This is precisely what was

zb — Wed, 14 Feb 2018 16:09:02 +0000

This is precisely what was going wrong. Many thanks, badlop! :)

Sidenote for others hitting this issue: 16.09 was actually working with a broken config as described, I don't why that's the case.

This happens because you

badlop — Wed, 14 Feb 2018 12:20:51 +0000

This happens because you configured ejabberd to serve several vhosts, like:

hosts:
  - "localhost"
  - "example.net"
  - "x3mpp.net"

But later, you configure the module with a fixed URL for the module in the three vhosts:

modules:
  mod_http_upload:
    put_url: "https://fixedurl:5444"

When ejabberd starts, it starts one instance of that module for each vhost. If the three instances of the module try to use the same URL, obviously the second and third will fail.

Solutions:
A) Have only one vhost
B) Define that option in a generic way, so it gets different for each vhost, as it was in the default value that used @HOST@
C) Configure that module differently for each vhost, see https://docs.ejabberd.im/admin/configuration/#virtual-hosting

ejabberd - Comments for "ejabberd 18.01 mod_http_upload already_started"

This is precisely what was

This happens because you