Here it is a detailed explanation on how to find/reproduce "the solution" (well, in this case, when you have already checked everything else is ok).
I discovered the problem by adding an error_reporting(8191) in the file websiteroot/connectors/index.php, just before the last lines which "handle the request":
error_reporting(8191);
/* handle the request */
$connectorRequestClass = $modx->getOption('modConnectorRequest.class',null,'modConnectorRequest');
$modx->config['modRequest.class'] = $connectorRequestClass;
$modx->getRequest();
$modx->request->sanitizeRequest();
This way, you can read the javascript error as the xhr response to the call modx makes to retrieve the assets tree.
In particular (for example using Chrome DevTools) check your network/xhr for a call to "directory.php?action=getList&id=/".
This way I found an error related to "mb_strtolower()".
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