OK, I've had a look and think I've worked it out. It's an issue with the theme.
Make sure you backup the following two files as I can't guarantee that this wont mess something else up as I haven't had long to test it:
admin/theme-settings.php
lib/widgets.php
Change line 20 in theme-settings.php from this:
<iframe id="themepreview" name="themepreview" src="<?php echo get_option('siteurl'); ?>/?preview=1"></iframe>
To this:
<iframe id="themepreview" name="themepreview" src="<?php echo get_option('home'); ?>/?preview=1"></iframe>
Change line 380 in widgets.php from this:
type: "GET",url: "<?php bloginfo('wpurl'); ?>",data: { widget_id: '<?php echo $id; ?>', twituser: '<?php echo $twituser; ?>', twitcount: '<?php echo $twitcount; ?>', get_twitter_data: 1 },
To this:
type: "GET",url: "<?php bloginfo('url'); ?>",data: { widget_id: '<?php echo $id; ?>', twituser: '<?php echo $twituser; ?>', twitcount: '<?php echo $twitcount; ?>', get_twitter_data: 1 },
It works, but I haven't done a lot of testing.
Rich