Using rebar3_ex_doc with an umbrella app: unknown error occurred generating docs config. Run with DIAGNOSTICS=1 for more details

I am trying to use the rebar3_ex_doc plugin to generate docs for the Erlang LS umbrella project.

The additional configuration I’m using is minimal:

{project_plugins, [rebar3_ex_doc]}.
{hex, [{doc, ex_doc}]}.

If I don’t specify any arguments when invoking the plugin:

rebar3 ex_doc

The plugin fails with:

An unknown error occurred generating docs config. Run with DIAGNOSTICS=1 for more details.

Enabling diagnostics does not give any additional useful information.

If I specify an application manually, as in:

rebar3 ex_doc -a els_lsp

The command exits with a 0 status code, but no documentation is produced.

What am I missing?