Error when saving changed in theme setttings

I've just moved my site from my local machine to a server.. currently at http://www.stroudsong.co.uk/d8 and when I change anything in the appearance->advanced settings I get WSOD. The site's ok apart from this error. I've tried upping the memory but that didn't help. you can see my pho info here if that helps : http://www.stroudsong.co.uk/d8/info.php

Fatal error: Class 'Drupal\at_core\Helpers\FileSavePrepare' not found in /home/sites/stroudsong.co.uk/public_html/d8/themes/adaptivetheme/at_core/forms/ext/advanced_settings_submit.php on line 17

help appreciate.

Am loving learning drupal 8 and the new adaptive themes... but struggling a bit with bugs (which I know isn't surprising at this stage). I know I'm not supposed to be building a production site yet... but I'm getting too impatient...

Ben