"Unable to connect to backend. Backend not found" seems to be a Javascript Exception? Maybe your options aren't complete. Possibibly you forgot to configure 'url' option within your view-helper code.
'url' options seems to configure backend route the ElFinder Javascript tries to connect to. Default route seems to be '/quelfinder/connector' ? (QuElFinder Module route config)
echo $this->QuElFinder('elfinder', array(
'width' => "50%",
'height' => '300',
'url' => '/quelfinder/connector'
)
);