Stack trace looks like its either a configurational or a permission issue. I think what's happening is that the endpoint invoked isn't returning the JSON body that the plugin expects. Rather, your installation of drupal is returning a no permission page which could be down to either the module not being activated correctly, the directory manager not pointing to the right url or that there's some drupal permission that we need to set in your case.
Lets do this. I'm attaching a simple html page that you can download and open on your browser. If I read the stack trace correctly, your localhost url is http://localhost/pqdemo (which should load your drupal home page). Just open the html attachment and then try logging in (Could be a bogus or a real login. Doesn't really matter in our case). If you see a simple json body that should say something along the lines of
{"user_login":"admin","login_success":"1"}
that'll mean that your module was correctly installed and activated. If you don't that means that we'll need to revisit your module installation.
Now if the above works lets check your plugin configuration. Your directory manager configuration should be pointing to http;//localhost/pqdemo (assuming again that I've read the trace correctly).
If both those check out it'll mean that drupal might be freaking out about remote posting and we'll move our troubleshoot there. From what I see in the error logs, I'm putting my money on either of the 2 above tho. Give it a go and lets see how that works out.
Have fun and good luck :)