I've seen, that there is one reference onto a local image, probably your developing-environment.
I found it inside PLUGIN_DIR/custom-sidebars/views/widgets.php in line 20.
<img src="http://local.wp33/wp-admin/images/wpspin_dark.gif" class="ajax-feedback" title="" alt="">
Would be nice, if you replace it with something working.
By the way, a really nice & cool Plugin.
Thanks for your work.
Best Regards Carsten