When the setting 'FilterHiddenFromDB' is enabled in ezfind.ini and //all// nodes are visible the Filter has an empty value and no search result is returned.
I fixed it like this in ezfind/classes/ezfezpsolrquerybuilder.php - line 166:

// only add filter if there are invisible nodes
if($invisibleNodeIDArray)
{
    $filterQuery[] = $hiddenNodesQueryText;
}

Steps to reproduce

set FilterHiddenFromDB=enabled in ezfind.ini

makes sure that //no// node is hidden

search...