https://docs.ejabberd.im/admin/guide/managing/#web-admin
You need to use full “user@domain.org” to authenticate. Make sure that admin acl is set for “user” not for “user@domain”, to work for all virtualhosts.
ejabberctl dump config
Look for file config and find lines:
{passwd,{<<"user1">>,<<"hostname.com">>},<<"secret1">>}.
{passwd,{<<"user2">>,<<"hostname.com">>},<<"secret1">>}.