control panel doesn't load

This is WordPress v3.3.1 with scheduler plugin v2.3.1
The plug looks like it’s correctly installed because it renders the calendar correctly in our website but I’m unable to change any settings with the plugin control panel because it never completely loads - when I go to Plugins->scheduler it just shows a small progress bar in the centre. Any ideas? Thanks

does any JavaScript error occur?
It may be cross-domain request error.