Answering my own question: it works with the following localsettings:
$wgEnableJS2system = true; $wgEnableScriptLoader = false; $wgEnableScriptMinify = false; $wgDebugJavaScript = false; $wgEnableScriptLoaderJsFile = true;
(and probably more combinations of the 2nd and later configuration variables)
The first variable was NOT yet mentioned on the documentation page for JS2, which caused my confusion.
Gregor