ejabberd reports bad request/modify when attempting to delete user - runtime-error

Question

I'm trying to delete user using ejabberd yet I'm receiving 400/modify bad-request error. The stanzas sent by Smack are by the spec [1] as far as I can tell; it also works fine with OpenFire.
Ejabberd runs in rroemhild/docker-ejabberd docker container version 17.03-1 (the latest available; for reasons why I am not using the processone container see [2]) with the config file being stock from there (sections removed; full copy at [3])
hosts:
- "xmppserver.com"
auth_method:
- internal
acl:
admin:
user:
- "admin": "xmppserver.com"
- "adminaccount": "xmppserver.com"
local:
user_regexp: ""
access:
(nothing of interest here)
modules:
mod_adhoc: {}
host_config:
"xmppserver.com":
domain_certfile: "/opt/ejabberd/ssl/xmppserver.com.pem"
(Yes, I will properly configure SSL once I verify it works for me)
First I confirm the user is authorised to issue admin commands:
SENT
<iq to='xmppserver.com' id='Ghtu9-15' type='get'>
<query xmlns='http://jabber.org/protocol/disco#items' node='http://jabber.org/protocol/commands'/>
</iq>
RECEIVED
<iq xml:lang='en' to='appserver#xmppserver.com/39058022031067483561501494598609310' from='xmppserver.com' type='result' id='Ghtu9-15'>
<query node='http://jabber.org/protocol/commands' xmlns='http://jabber.org/protocol/disco#items'>
...
<item node='http://jabber.org/protocol/admin#add-user' name='Add User' jid='xmppserver.com'/>
<item node='http://jabber.org/protocol/admin#delete-user' name='Delete User' jid='xmppserver.com'/>
...
</query>
</iq>
As you can see the delete-user command is listed as available. Then I proceed to delete a user:
SENT
<iq to='xmppserver.com' id='2vM3f-17' type='set'>
<command xmlns='http://jabber.org/protocol/commands' action='execute' node='http://jabber.org/protocol/admin#delete-user'></command>
</iq>
RECEIVED
<iq xml:lang='en' to='adminaccount#xmppserver.com/163923254889085958281501260654263394' from='xmppserver.com' type='result' id='2vM3f-17'>
<command status='executing' sessionid='2017-07-28T16:51:12.784282Z' node='http://jabber.org/protocol/admin#delete-user' xmlns='http://jabber.org/protocol/commands'>
<actions execute='complete'>
<complete/>
</actions>
<x type='form' xmlns='jabber:x:data'>
<title>Delete User</title>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/admin</value>
</field>
<field var='accountjids' type='jid-multi' label='Jabber ID'>
<required/>
</field>
</x>
</command>
</iq>
Here I'm not sure what the <actions...> tag is about; it's not mentioned in XEP-133. Other than that the form looks good.
SENT
<iq to='xmppserver.com' id='2vM3f-19' type='set'>
<command xmlns='http://jabber.org/protocol/commands' sessionid='2017-07-28T16:51:12.784282Z' node='http://jabber.org/protocol/admin#delete-user'>
<x xmlns='jabber:x:data' type='submit'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/admin</value>
</field>
<field var='accountjids' type='jid-multi'>
<value>jid#xmppserver.com</value>
</field>
</x>
</command>
</iq>
RECEIVED
<iq xml:lang='en' to='adminaccount#xmppserver.com/163923254889085958281501260654263394' from='xmppserver.com' type='error' id='2vM3f-19'>
<command sessionid='2017-07-28T16:51:12.784282Z' node='http://jabber.org/protocol/admin#delete-user' xmlns='http://jabber.org/protocol/commands'>
<x type='submit' xmlns='jabber:x:data'>
<field var='FORM_TYPE' type='hidden'>
<value>http://jabber.org/protocol/admin</value>
</field>
<field var='accountjids' type='jid-multi'>
<value>jid#xmppserver.com</value>
</field>
</x>
</command>
<error code='400' type='modify'>
<bad-request xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
</error>
</iq>
I also tried sending bare JIDs (i.e. without the hostname part) but the result is the same.
This is not a particularly helpful error message.
What am I doing wrong?
[1] https://xmpp.org/extensions/xep-0133.html#delete-user
[2] https://github.com/processone/ejabberd/issues/1890
[3] https://pastebin.com/5D0rEbFR

Answer 1

Apparently <error code='400' type='modify'> is eJabberD's way of saying 'user not found, nothing to delete'.